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

THE CHARACTERISTICS OF GAS HYDRATES FORMED FROM H2S AND CH4 UNDER VARIOUS CONDITIONS Schicks, Judith M.; Lu, Hailong; Ripmeester, John A.; Ziemann, Martin 2008-07-31

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

Item Metadata

Download

Media
59278-5712.pdf [ 139.32kB ]
Metadata
JSON: 59278-1.0040964.json
JSON-LD: 59278-1.0040964-ld.json
RDF/XML (Pretty): 59278-1.0040964-rdf.xml
RDF/JSON: 59278-1.0040964-rdf.json
Turtle: 59278-1.0040964-turtle.txt
N-Triples: 59278-1.0040964-rdf-ntriples.txt
Original Record: 59278-1.0040964-source.json
Full Text
59278-1.0040964-fulltext.txt
Citation
59278-1.0040964.ris

Full Text

THE CHARACTERISTICS OF GAS HYDRATES FORMED FROM H2S AND CH4 UNDER VARIOUS CONDITIONS  Judith M. Schicks* GeoForschungsZentrum Potsdam Telegrafenberg, 14473 Potsdam GERMANY  Hailong Lu, John A. Ripmeester The Steacie Institute for Molecular Sciences National Research Council of Canada 100 Sussex Drive, Ottawa, Ontario, K1A0R6 CANADA  Martin Ziemann Institut f?r Geowissenschaften Universit?t Potsdam, 14476 Potsdam GERMANY  ABSTRACT Shallow marine gas hydrates occurring above the Sulfate-Methane-Interface (SMI) often contain small  amounts  of  H2S  beside  methane  and  other  hydrocarbons,  but  the  distribution  of  H2S  in these natural samples is not always homogeneous. To learn more about the formation of H2S-containing  hydrates,  gas  hydrates  with  different  ratios  of  H2S/CH4  were  synthesized  under various conditions. The samples were synthesized from ice and water phases, with constant feed gas compositions or controlled changes in feed gas compositions. It turns out that the detailed nature  of  the  synthetic  hydrate  samples  depends  on  the  method  of  sample  preparation.  The sample prepared with gas containing small amounts of H2S (1% H2S and 99% CH4) appeared homogeneous in composition, while that prepared in a water-H2S-CH4 system with higher H2S contents  was  heterogeneous.  The  samples  were  analysed  with  Raman  spectroscopy,  and differential scanning calorimetry (DSC).  Keywords: H2S-CH4-hydrates, differential scanning calorimetry, Raman spectroscopy    INTRODUCTIONdotmath Recent  investigations  on  shallow  marine  gas hydrates  recovered  from  IODP  Expedition  311 provide data regarding the distribution of H2S and methane  in  these  natural  samples.  It  turned  out, that even in hydrate pieces that appeared as large, homogeneous and transparent crystals, the spatial distribution  of  composition  varied  [1].  The heterogeneous  compositions  of  these  natural samples may be the result of a hydrate formation process  in  an  environment  with  fluctuating composition of H2S and CH4 in the feed gas over a temporal and/or spatial scale. In shallow marine sediments,  H2S  is  locally  produced  by  the reduction  of  sulphate  via  anaerobic  methane oxidation  (AMO)  as  a  result  of  a  complex interaction  between  microbes  ?  a  syntrophic consortium  of  methanotrophic  archaea  and sulphate  reduction  bacteria  -  which  uses  the sulphate  to  oxidize  the  methane  anaerobically [2,3,4,5]. The methane is in general from deeper sediment  sections,  either  originally  biogenic  or thermogenic. Both gases in hydrate are related to the hydrocarbon flux, and the ratio between them might  change  over  time.  The  hydrate  formation and growth is supported by the dissolved gases in the  surrounding  pore  water.  Therefore,  the hydrate  will  only  incorporate  gases  into  its structure  which  are  available  in  the  pore  water. But  it  is  questionable  if  the  composition  of  the hydrate  changes  or  homogenization  will  happen for the resulting hydrate when the composition of gas available from pore water changes. Experimental  data  about  formation  and  growth processes for hydrates containing H2S beside CH4 are rare. Noaker and Katz studied the conditions Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  *Corresponding author: Phone: +493312881487 Fax: +493312881474 E-mail: schick@gfz-potsdam.de for hydrate formation with hydrogen sulphide and methane-water  mixtures  but  they  did  not investigate  the  compositions  of  the  resulting hydrate phases [6]. Others, such as Selleck et al. or Carroll and Mather investigated the behaviour of hydrogen sulphide?water mixtures and in this context  the  H2S-hydrate  formation  conditions [7,8].  Robinson  and  Hutton  reported  hydrate formation  in  the  systems  containing  CH4,  H2S and CO2 [9]. However, it is very likely that the phase  behaviour  and  hydrate  formation  in  those systems containing three gas components beside water will differ from a system containing CH4, H2S and H2O. To learn more about the formation of  H2S-CH4-hydrates  and  the  complex interactions  mentioned  before,  we  performed experiments  with  gas  hydrates  which  were synthesized from water or ice and different H2S-CH4-compositions.   EXPERIMENTAL METHODS Gas hydrates were synthesized from water or ice and  gas  mixtures  containing  H2S  and  CH4  in various  concentrations  with  different  synthesis routes  and  were  characterized  with  Raman spectroscopy  or  Differential  Scanning Calorimetry (DSC).   In situ Raman spectroscopic measurements: For  Raman  spectroscopic  measurements  the  gas hydrates  were  synthesized  in  a  pressure  cell which  has  been  described  in  detail  elsewhere [10].  The  pressure  cell  can  be  used  in  a temperature range between 245 K and 350 K. The temperature of the sample cell is controlled by a thermostat  and  the  temperature  is  determined with  a  precision  of  ?0.1  K.  The  applicable pressure range is between 0.1 and 10.0 MPa. A pressure  controller  adjusts  the  pressure  with  a precision of 2% relative.  The  experiments  were  performed  with  a continuous gas flow to avoid changes in the gas composition.  With  a  gas  flow  of  1  ml/min,  it takes 17 sec for the incoming gas to pass the cold cell  body  and  to  enter  the  cell  void  space;  this time is sufficient to allow the gas flow to attain the cell temperature. Therefore the sample in the cell is cooled from cell body below and the gas phase  above.  Please  note,  that  the  small  sample volume  (393  ?l)  and  the  above  mentioned  all-around  cooling  of  the  sample  prevent  the possibility of establishing a temperature gradient. A  quartz  window  permits  the  analysis  of  the phases by Raman Spectroscopy as well as visual observation  and  the  recording  of  microscopic photo-documentation  of  formation  and decomposition processes. The  experiments  were  carried  out  using  the following  procedure.  First,  150  ?l  pure  and degassed water or 100 ?g ice were placed in the sample cell. In case of hydrate synthesis from the liquid water phase, the cell was carefully sealed and  flushed  with  the  appropriate  gas  before pressurization  (1%  H2S  and  99%  CH4). Thereafter,  the  system  was  cooled  down  as rapidly  as  possible  until  hydrates  were  formed. After  that,  the  system  was  warmed  at  constant pressure  in  order  to  melt  most  of  the  hydrate. When  only  a  few  crystals  were  left,  the temperature  was  lowered  by  0.5  K  and  the euhedral  crystals  of  gas  hydrate  grew  under steady state conditions. In case of hydrate synthesis from the ice phase, the  ice  was  placed  into  the  sample  cell  at  a temperature of 272 K and slowly pressurized. The  Raman  spectra  were  taken  with  a  confocal Raman  spectrometer  (LABRAM,  HORIBA JOBIN YVON), which allowed the laser beam to be focused on an exact point, e.g. the surface of a hydrate  crystal,  thus  assuring  that  only  the selected phase was analyzed.  Hydrate  sample  preparation  for  Differential Scanning  Calorimetry  (DSC)  and  Raman Spectroscopy  after  hydrate  formation  from ice-H2S-CH4 or water-H2S-CH4 The  CH4-H2S-hydrates  were  synthesized  from powdered  ice  with  gas  composition  of  1%  H2S and 99% CH4. 5 g powdered ice was filled into a pressure  vessel  at  263  K.  The  vessel  itself contained a volume of approximately 1.1 L. The vessel  was  carefully  sealed  and  pressurized  (9  MPa). The  pressure  vessel  was  placed into  a cooling box at 263 K for several weeks until no more  changes  in  pressure  were  observed.  The samples  were  recovered  from  the  accumulators and stored immediately in liquid nitrogen. Before analyses,  the  gas  hydrate  samples  were  handled in  liquid  nitrogen  inside  a  nitrogen-flooded cooled glovebox. To  examine  whether  composition homogenization  happens  or  not  after  initial hydrate  formation,  a  H2S-CH4  hydrate  sample was also prepared from a water-H2S-CH4 system. For the synthesis 7 cm3 of pure water was added to a 125 cm3 pressure cell. After evacuation of the pressure cell for 20 minutes H2S and CH4 gases were charged into the cell at a ratio of 1:2 (v:v) to 6  MPa.  Thereafter  the  cell  was  put  into  a  276.15  K  water  path  to  let  the  gas  react  with water. When no obvious change in pressure was observed,  the  sample  prepared  was  ready  for recovery. For sample recovery, the pressure cell was first cooled in dry ice, and then the gas was released. Finally the sample was recovered while the cell sat in a liquid nitrogen bath. The sample recovered was stored in a liquid nitrogen dewar for characterization.  RESULTS AND DISCUSSION In-situ Raman spectroscopic measurements: One  series  of  experiments  have  been  performed to investigate the formation and growth processes of  H2S-CH4-hydrates  regarding  the  formation kinetics,  cage  occupancy,  composition  of  the resulting  hydrate  phase  and  its  stability  fields. The  Raman  spectroscopic  measurements  have been  performed  in-situ  during  the  hydrate formation from liquid water and the gas phase. It turned  out  that,  compared  to  pure  methane hydrate, the formation of a solid phase was much faster, but the resulting crystals did not show the expected Raman spectra of a structure I hydrate phase.  Representative  spectra  from  this  early formation of crystals are shown in Figure 1.    Figure  1  Raman  spectra  of  preliminary  hydrate crystals formed from water and gas phase. At the top:  C-H  stretching  bands  at  2903  cm-1  and  2815  cm-1  and  O-H  stretching  bands.  At  the bottom: S-H stretching band at 2593 cm-1.  Two  bands  at  2903  cm-1  and  2915  cm-1  were detected  and  assigned  to  C-H  nu1  stretching modes  for  CH4  in  large  51262  and  small  512 cavities,  respectively.  Also,  typical  O-H stretching  bands  were  detected  in  the  range between  3000  cm-1  and  3600  cm-1.  In  addition, one band at 2593 cm-1 was detected and assigned to S-H nu1 stretching mode for H2S in large 51262 cavities of structure I hydrate, whereas there was no obvious indication for H2S in small cavities of structure  I  hydrates  from  the  Raman  spectra  at this point of time. With  time,  these  crystals  transformed  and exhibited the typical Raman spectra for structure I hydrate after approximately 48 h. These Raman spectra  are  shown  in  Figure  2.  This transformation  from  crystals  with  unclear structure  to  those  with  structure  I  character proceeded without obvious changes regarding the appearance of the crystals. All detected bands are listed in Table 2.      Figure 2 Raman spectra of the hydrate phase after 48 h. At the top: complete spectra. Middle: S-H stretching band at 2593 cm-1 and 2602 cm-1 and CH4  bending  (2568  cm-1).  At  the  bottom:  C-H stretching bands at 2903 cm-1 and 2915 cm-1.  Table 2. Detected bands and assignment. Band position [cm-1] Component  Band assignment 2568  CH4  2nu4 (bending) 2593  H2S  nu1(S-H ) 2602  H2S  nu1(S-H ) 2903  CH4  nu1(C-H ) 2915  CH4  nu1(C-H) *: the band position may vary for 1 cm-1  It turned out that the composition of the hydrate phase  differs  significantly  from  that  of  the  feed gas.  The  resulting  hydrate  phase  was  almost homogeneous.  The  average  composition  of  the gas in the hydrate phase was 12% H2S and 88% CH4,  whereas  the  composition  of  the  feed  gas was 1% H2S and 99% CH4. The stability of these hydrogen  sulphide  containing  hydrates  was shifted  to  slightly  higher  temperatures  or  lower pressures  compared  to  pure  methane  hydrate  as shown in Figure 3.   Figure 3 Decomposition line for CH4-hydrate and H2S-CH4-hydrate.   Based  on  this  observation  the  question  arose  if the preferred incorporation of H2S into the large 51262 cavities of structure I hydrate at the initial stage  was  typical  of  the  formation  and  growth process  for  H2S-containing  hydrates  or  if  this observation  was  a  result  of  the  spontaneous formation of ice and hydrate crystals during the first cooling period due to the fact that no hydrate formation  was  observed  from  water  and  gas without sub-cooling the system until spontaneous hydrate  and  ice  formation  occurred.  To  clarify this, another run of experiments was performed: the  time-depending  observation  of  hydrate formation and growth from ice and a gas phase. These experiments were performed at 272 K and 2.1  MPa.  15  min  after  pressurisation  of  the sample  cell  filled  with  powdered  ice  the formation of a gas hydrate layer at the surface of the ice crystal was observed. The formation and growth  processes  were  documented  during  the following  six  hours.  Raman  spectra  were  taken every 15 min. The results are presented in Figure 4 and Figure 5. Figure  4  shows  the  development  of  the  Raman spectra regarding the nu1 stretching mode of CH4 for methane encased in large 51262 and small 512 cavities  of  structure  I.  At  the  beginning  of  the experiment,  only  one  band  at  2915cm-1  was detected  and  assigned  to  CH4  encaged  in  the small 512 cavities. After 45 min a second band at 2903 cm-1 was detected, indicating CH4 encased in the large hydrate cavities. The intensity of this band increased with time indicating an increasing incorporation of CH4 into the large 51262 cavities and  thus  the  transformation  of  the  preliminary structure into structure I with time.   Figure  4:  Real-time  Raman  spectra  monitoring the incorporation of CH4 in large 51262 and small 512 cavities of structure I.  Figure  5  shows  the  development  of  the  Raman spectra regarding the nu1 stretching modes of H2S encased in 51262 and 512 cavities of structure I. At the beginning of the experiment no indication for H2S encased into the solid phase was given from the Raman spectra. After 60 min only one band at 2592  cm-1  was  detected, indicating  H2S  in  51262 cavities of structure I. After 210 min the ratio of the integral intensities of the bands at 2592 cm-1 to  2602  cm-1  reached  3:1  which  corresponds  to the  ratio  of  large  51262  to  small  512  cavities  in structure I. These experimental results lead to the conclusion that at the beginning of the hydrate formation and growth process the H2S is preferentially encased into  the  large  cages  whereas  CH4  is  primarily encaged into the small cages. With time the ratio of  the  integral  intensities  of  the  bands  for  both, H2S and CH4 reached 3:1 which corresponds well to the ratio of large to small cavities in structure I. The  resulting  hydrate  phase  in  the  equilibrium state  is  a  mixed  H2S-CH4-hydrate  with homogeneous  composition  where  both  gas molecules  occupy  large  51262  and  small  512 cavities of structure I.   Figure  5:  Real-time  Raman  spectra  monitoring the  incorporation  of  H2S  in  large  and  small cavities. A third series of experiments has been performed to study the exchange reaction of H2S with CH4 in case of a change in the composition of the gas phase.  For  these  experiments  150 ?l  pure  water was  placed  into  the  sample  cell.  The  cell  was sealed  and  flushed  with  the  pure  methane  gas (99.995  %  CH4).  Thereafter,  the  system  was cooled down as rapidly as possible until hydrates (and ice) were formed. After that, the system was warmed at constant pressure in order to melt the ice  and  most  of  the  hydrate.  When  only  a  few crystals  were  left,  the  temperature  was  lowered by  1  K  and  the  euhedral  crystals  of  methane hydrate  grew  under  steady  state  conditions  for one  week.  The  resulting  hydrate  phase  was analysed with Raman spectroscopy, showing the prominent  bands  at  2903  cm-1  and  2915  cm-1 indicating  the  incorporation  of  CH4  into  large 51262 and small 512 cavities of structure I hydrate. The ratio of the integral intensities of the bands at 2903 cm-1 and 2915 cm-1 was approximately 3:1, corresponding to the ratio of large 51262 to small 512 cavities in structure I. At that point of time the composition  of  the  gas  phase  was  changed  into 1%  H2S  and  99%  CH4  without  changing  the pressure  or  temperature  conditions.  Raman spectra were taken over the next seven days. No change in composition of the hydrate phase could be  detected  by  Raman  spectroscopy.  This  result implied  that  the  methane  hydrate  has  not  been affected by H2S after it was formed.  Differential Scanning Calorimetry (DSC) and Raman spectroscopy  The  results  from  DSC  measurements  generally were  not  self  consistent.  It  turned  out  that  the hydrates, formed from ice and gas (1% H2S-99% CH4) showed only one endothermic event (beside melting  of  ice  at  273.15  K)  with  an  onset temperature  of  TO  =  236.7  K  ?  1.2K indicating the dissociation of a H2S-CH4 mixed hydrate with a  homogeneous  composition  into  ice  and  gas.  In  contrast,  as  shown  in  Figure  6,  the  Raman spectroscopic  analysis  of  the  hydrate  sample formed  from  water-H2S-CH4  (H2S:CH4  =  1:2) indicated  an  inhomogeneous  composition,  and the  DSC  thermogram  showed  a  broad  range  of dissociation  of  H2S-CH4  hydrate  with  an  onset-temperature of TO = 246 K (see also Figure 7). As a result the hydrate sample formed in the water-H2S-CH4  system  showed  the  coexistence  of different  hydrate  phases  with  varying composition, similar to the phenomenon observed for natural gas hydrates [11]. The distribution of compositions in the heterogeneous sample might have resulted from the composition change due to the  preferentially  incorporating  of  H2S  into  the hydrate  phase  throughout  the  reaction  progress. Because  the  reaction  in  this  system  lasted  23 days, no significant composition homogenization was observed for the hydrate formed, consistent with the results of in situ observations.    Figure  6:  The  Raman  spectra  of  H2S-CH4 hydrates formed in Water-H2S-CH4 system.  2600 2700 2800 2900Raman shift (cm-1)99% 1% 94%  6% 92% 8% H2S CH4  Figure  7:  The  DSC  thermogram  of  H2S-CH4 hydrate prepared in a water-H2S-CH4 system.  CONCLUSIONS Raman spectroscopic measurements on H2S-CH4-hydrates  formed  from  a  gas  phase  and  liquid water or ice showed that during the initial stages of  hydrate  formation  H2S  was  preferentially incorporated  into  the  large  51262  cavities  of structure  I.  In  contrast,  CH4  was  preferentially encased into the small 512 cavities of structure I during the initial stages of hydrate formation. The resulting hydrate phase showed the occupancy of large  51262  and  small  512  cavities  of  structure  I with  both  guest  molecules,  H2S  and  CH4  in  the same ratio (3:1). This hydrate phase, formed from a  gas  mixture  containing  only  1%  H2S  beside CH4 showed a homogeneous composition where H2S was strongly enriched into the hydrate phase. The  resulting  composition  of  the  gas  phase encased in the hydrate lattice was 12 % H2S and 88 % CH4. In  case  of  hydrate formation  from  a  water-H2S-CH4 with higher content of H2S in the feed gas both  Raman  and  DSC  measurements  indicated the  formation  of  multiple  hydrate  phases  with different compositions.                                                    REFERENCES:  [1] Schicks J.M., Ziemann M. A., Lu H., Ripmeester J.A. Raman spectroscopic investigations on natural samples from IODP Expedition 311: indications for heterogeneous compositions in hydrate crystals  Geophysical research Letters, submitted                                                                             [2] Barnes R.O., Goldberg E.D. Methane production and consumption in anoxic marine sediments. Geology  1976, 4: 297-300  [3] Zehnder A.J.B., Brock T.D. Methane formation and methane oxidation by methanogenic bacteria. J. Bacteriol. 1979, 137: 420-432  [4] Kastner M., Kvenvolden K. A., Lorenson T. D. Chemistry, isotopic composition, and origin of a methane-hydrogen sulfide hydrate at the Cascadia subduction zone. Earth and Planetary Science Letters 1998, 156: 173-183.  [5] Boetius A., Ravenschlag K., Schubert C.J., Rickert D., Widdel F., Giesecke A., Amman R., J?rgensen B.B., Witte U., Pfannkuche O. A marine microbial consortium apparently mediating anaerobic oxidation of methane. Nature 2000, 407: 623-626  [6] Noaker L.J., Katz D.L., Gas Hydrates of Hydrogen Sulfide-Methane Mixtures, Petrolium Transactions, AIME 1954, 201, 237-239   [7] Selleck F.T., Carmichael L.T., Sage B.H. Phase Behaviour in the Hydrogen Sulfide-Water System. Industrial and Engineering Chemistry 1952, 2219-2226   [8] Carroll J.J., Mather A.E. Phase Equilibrium in the System Water-Hydrogen Sulphide: Hydrate-Forming Conditions. The Canadian Journal of Chemical Engineering 1991, 69:1206-1212   [9] Robinson D.B:, Hutton J.M. Hydrate Formation in Systems Containing Methane, Hydrogen Sulphide and Carbon Dioxide. The Journal of Canadian Petroleum Technology 1967, January ?March: 6-9  [10] Schicks J.M., Ripmeeste, J.A. The coexistence of two different methane hydrate phases under moderate pressure and temperature conditions: kinetic vs thermodynamic products. Angewandte Chemie International Edition 2004, 43:3310-3313.  [11] Lu, H., Schicks, J.M., Moudrakovski, I.L., Udachin, K., Ripmeester, J.A., Zhang, M., Naumann, R., Dutrisac, R., Luzi, M., and Erzinger, J. Laboratory-based characterization of gas hydrate samples recovered from northern                                                                            Cascadia by IODP Expedition 311. Journal of Geophysical Research, submitted.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.59278.1-0040964/manifest

Comment

Related Items