Natural Gas Hydrates Jakob de Swaan Arons Professor Royal Dutch Shell Chair Chemical Engineering...
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Transcript of Natural Gas Hydrates Jakob de Swaan Arons Professor Royal Dutch Shell Chair Chemical Engineering...
Natural Gas Hydrates
Jakob de Swaan Arons
Professor
Royal Dutch Shell Chair
Chemical Engineering Department
Tsinghua University, Beijing, China
19th September 2006
Shell – Tsinghua Chair Professorship
Royal Dutch Shell
Shell Transport and Trading Company (British)
and
Royal Dutch Petroleum Company (Dutch)
Contents
What are gas hydrates Models, thermodynamics and phasebehavior Applications Conclusions
What is a Gashydrate?
Gashydrates
Crystalline structures of water with cavities of molecular size, containing (hosting) molecules of compounds (guest) with boiling points mainly below and sometimes above room temperature.
What are gashydrates?
Water Molecule
Hydrate bonding H20 molecule
Guest molecule:
CH4, C2H6, i-C4H10,CO2, N2, O2, CHF3
Interaction and Stability512 cavity Ice structure: more stable
T = 273 K
Hydrogen bonding H2O molecules
C
H H
HH Interaction between guest and H2Omolecules stabilizes the structure
Various Gashydrate Structures
Structure H
Structure I Structure II
34 water molecules
136 water molecules46 water molecules
Uit: E.D. Sloan Jr., Hydrate Engineering,Bloys, B. (ed.), SPE Monograph Series,21, Richardson, Texas, V.S., 2000
Compare size of molecule and cavity
512 [sI]
51262 [sI]
51264 [sII]
51268 [sH]
435663 [sH]
512 [sII, sH]
4 Å
5 Å
6 Å
7 Å
8 Å
4 Å
5 Å
6 Å
7 Å
8 Å
N2 O2
CH4
CH3
CO2
C2H6
CF4 OC3H8
O
O O
Gashydrate flame
Importance
Nuisance Blessing ? Separations Scientific
Formation of a hydrate plug
vapour
oil & water
vapour
oil & water
hydrate hydrate
oil & water
vapour
Hydrate crystals Hydrate plug
They resemble ice, we find them in Nature.
Gas hydrateGas
Natural gas hydrate reservoirs
Locates Gas Hydrate:Zeebodem PermafrostHydrate
10.000Fossil 5.000
Other3.780
K.A. Kvenvolden, A Primer on the Geological Occurrence of Gas Hydrate, in: Gas Hydrates – Relevance to World Margin Stability and Climate Change, Henriet, J.-P., Mienert, J. (eds.), Geol. Soc. Special Publ., 137, Geological Society, Londen, GB, p. 9-30, 1998
Hydrate RidgeBlake Ridge
Noorse ZeeBarents Zee Zee van Okhotsk
McKenzieDelta
PrudhoeBay
Stability and natural conditionsD
epth
van
sed
imen
t [m
]
Temperature [°C]
0
200
400
600
800
1000
1200
1400
16000 10 20 30-10-20
Wat
er D
epth
[m]
Temperature [°C]
0
200
400
600
800
1000
1200
1400
16000 10 20 30-10-20
dieptepermafrost
geothermalgradient
fasen begrenzing
basisgas hydrate
stablegas hydrate
watersediment
hydrate-mischegradient
geothermalgradient
Phase boundary
basis gas hydrate
stable gas hydrate
Permafrost Oceaan
Industrial question
Dutch natural gas may contain up to 14% N2. Could hydrates act as a good separation agent?
Scientific importance
As we will see later, gas hydrates offer an extremely interesting example of a large family of so called inclusion compounds made up of host- and guest- molecules.
Water Urea Hydroquinone
Question
What has the subject of Natural Gas Hydrates (NGH) to do with a course in
Advanced
Chemical Engineering
Thermodynamics?
Answer
In dealing with NGH we can demonstrate the power and beauty of Applied, Molecular and Statistical Thermodynamics.
Classical thermodynamics
Presents broad relationships between macroscopic properties but it is not concerned with quantitative prediction of these properties.
Example
John M. Prausnitz
( ) ( )T PH V
V TP T
Statistical thermodynamics
Seeks to establish relationships between macroscopic properties and intermolecular forces and other molecular properties.
Example
John M. Prausnitz
( ) / 2
02 [1 ]u r kT
AB N e r dr
Molecular thermodynamics
Seeks to overcome some of the limitations of both classical and statistical thermodynamics. It is an engineering science, based on classical thermodynamics but relying on molecular physics and statistical thermodynamics ……. .
In application it is rarely exact and has an empirical flavour.
John M. Prausnitz
Van der Waals – Platteeuw model for gashydrates
These former colleagues at Shell Research International developed a wonderful model, back in the 1950-ies, that since then has seen many small modifications but still “stands as a rock”.
Johan van der Waals and Joost Platteeuw
Adv. Chem. Phys. 2, 1-57 [1959]
Assumptions for the model
1. Guest molecules don’t affect the cavity structure
2. At most one guest molecule/cavity
3. No interactions between guest molecules
4. Guest molecule can rotate freely in cavity
5. Lennard- Jones type potential for interaction between guest and cavity
Interaction potential guest and cavity
2 0 2
0kB
a
u(r)
< R >
Intermolecular potential (1)
( ) ( ; , , , )u r u r a R
a = radius guest
R = radius cavity
r = variable distance from center cavity
= r value for which potential is 0
= potential at maximum attraction
Intermolecular potential (2)
The most successful potential has been proposed by the Japanese scientist Kihara. Its parameters have been optimized from experimental data on hydrate phase equilibria.
The “ Langmuirconstant” Cki
This constant can be expressed for guest k in cavity of type i by
2
0
4 ( )exp
R a
kiki
u rC r dr
kT kT
Guest k in cavity i
ˆ
ˆ1ki k
kiki k
C f
C f
ki expresses the fraction of cavity type i occupied by guest k.
In case of CH4 the fugacity is approximately the total pressure P
ˆkf
Cavity occupancy and host water
ˆˆ
ˆ (1 )1ki k ki
ki ki kki kiki k
C ffCC f
, ln(1 )H H emptyW W i ki
kRT v
“Langmuir”
“Raoult”
In case of water CH4 : the higher the gas pressure, the higher the cavity occupancy, the more stable the hydrate
vi= number of cavities type i
Analogies
The equations for the thermodynamic potential or fugacity of solutes (guests) and solvent (host) show a remarkable resemblance with those for adsorption (Langmuir) and solvency (Raoult).
Phase diagram of water
P
T
A
D
S
B
C
V
l sw wl vw wv l sw w w
A
B
DL
How gashydrate may “ take over” from ice below the melting point
…… empty ……
ice ……
filled
increase
CH4-pressure
How gashydrate may form from liquid water above the melting point
…… empty …… empty
…… ice ……
water ……
filled
Increasing
CH4- pressure
Freezing point depression (FPD)
…… l …… l
ice …… ice
l
add
FPD- agent
Methanol
Ethylene glycol
Salt?
Hydrate inhibition
Just like an FPD- agent is effective in suppression of ice formation it may suppress hydrate formation
…… empty H ……
…… liquid ……
…… ice ……
filled H ……
liquid
A costly “ affair”
Hydrate promoters (1)
, ln(1 )H H emptyW W i ki
i kv
Certain molecules, like tetrahydrofuran (THF), may promote hydrate formation by assisting in filling the vacancies.
Hydrate Promoters (2)
These, usually non-volatile, promoters may produce the hydrate structure in which the gas molecules can be included although their pressure is too low to achieve this by themselves.
(e.g. H2)
Solution?
It is my impression that these days industry employs inhibitors that don't suppress hydrate formation but suppress hydrate crystal growth producing some kind of “hydrate milk” that does not block pipeline operation.
“If you can’t beat the enemy, join them……”
Prediction (1)
In the oil-and gas industry one likes to know when hydrate formation can be expected, especially at temperatures above 0 °C.
Possible phases Hydrate H Liquid W aqueous Liquid non-aqueous Vapour V
Prediction (2)
Components of natural gas:
C1 C2 C3 …… N2 CO2 ……
For example: where is the location of the HLwV-equilibrium curve?
k = 1,2,……N
Models required for the various phases
( )
( )
lH v lww w w w
lH v lwk k k k
Prediction (3)
These days the large oil- and gas companies make use of powerful software to allow them to predict not only all possible hydrate phase diagrams but also the effect of inhibitors (by including for example methanol in the calculation programme).
Equilibrium conditions for different hydrocarbons
I-H-VH-Lw-VH-Lw-L
red CH4
blue C2H6
green C3H8
magenta i-C4H10
260 270 280 290
Temperatuur [K]
0
2
4
6
Dru
k [M
Pa]
Pre
ssur
e
Temperature [K]
Equilibrium conditions for some other gases
260 270 280 290 300 310
Temperatuur [K]
0
5
10
15
20
25
Dru
k [M
Pa]
I-H-V
H-Lw-V
H-Lw-L
H-L-V
red N2
blue CO2
magenta H2S
Pre
ssur
e
Temperature [K]
Influence salts, organic compounds
Temparature
log
Pre
ssu
re
H2O + CH4
H - Lw - V
Concentration
H2O + CH4 + NaClofH2O + CH4 + MeOH
H2O + CH4 + cyclicorganic component
Application: desalination
sea water
CO2 or air
brine
gas recycle
Hydrate formation
Separation
drink H2O
decomposition
Threat
CH4 is a much more serious contributant to greenhouse effect than CO2. So with the Earth warming up, natural gas hydrates may start dissociating and we may face a “runaway” greenhouse effect.
Also: Leaking pipelines in former Soviet-Union
Living on NGH ?
US Geological Survey [1998]
May I introduce myself ?
Acknowledgment
I wish to acknowledge the contributions of my former Ph. D. student Miranda Mooijer- Van den Heuvel, who is now with Shell Global Solutions International. She graduated on a thorough study of how certain compounds can promote hydrate formation.