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.