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 H 2 0 molecule Guest molecule: CH 4, C 2 H 6, i-C 4 H 10, CO 2, N 2, O 2, CHF 3

Interaction and Stability 5 12 cavity Ice structure: more stable T = 273 K Hydrogen bonding H 2 O molecules C H H H H Interaction between guest and H 2 O molecules stabilizes the structure

Various Gashydrate Structures Structure H Structure I Structure II 34 water molecules 136 water molecules 46 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 5 12 [sI] [sI] [sII] [sH] [sH] 5 12 [sII, sH] 4 Å 5 Å 6 Å 7 Å 8 Å 4 Å 5 Å 6 Å 7 Å 8 Å N 2 O 2 CH 4 CH 3 CO 2 C2H6C2H6 CF 4 O C3H8C3H8 O O O

Gashydrate flame

Importance  Nuisance  Blessing ?  Separations  Scientific

Formation of a hydrate plug vapour oil & water vapour oil & water hydrate oil & water vapour Hydrate crystals Hydrate plug

They resemble ice, we find them in Nature. Gas hydrate Gas

Natural gas hydrate reservoirs Locates Gas Hydrate: Zeebodem Permafrost Hydrate Fossil Other 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 Ridge Blake Ridge Noorse Zee Barents Zee Zee van Okhotsk McKenzie Delta Prudhoe Bay

Stability and natural conditions Depth van sediment [m] Temperature [°C] Water Depth [m] Temperature [°C] diepte permafrost geothermal gradient fasen begrenzing basis gas hydrate stable gas hydrate water sediment hydrate- mische gradient geothermal gradient Phase boundary basis gas hydrate stable gas hydrate Permafrost Oceaan

Industrial question Dutch natural gas may contain up to 14% N 2. 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

Statistical thermodynamics Seeks to establish relationships between macroscopic properties and intermolecular forces and other molecular properties. Example John M. Prausnitz

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  kBkB  a u(r)

Intermolecular potential (1) 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” This constant can be expressed for guest k in cavity of type i by

Guest k in cavity i expresses the fraction of cavity type i occupied by guest k. In case of CH 4 the fugacity is approximately the total pressure P

Cavity occupancy and host water “Langmuir” “Raoult” In case of water CH 4 : the higher the gas pressure, the higher the cavity occupancy, the more stable the hydrate v i = 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 A B D L

How gashydrate may “ take over” from ice below the melting point …… empty …… ice …… filled increase CH 4 - pressure

How gashydrate may form from liquid water above the melting point …… empty …… ice …… water …… filled Increasing CH 4 - pressure

Freezing point depression (FPD) …… 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) 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. H 2 )

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: C 1 C 2 C 3 …… N 2 CO 2 …… For example: where is the location of the HL w V- equilibrium curve? k = 1,2,……N Models required for the various phases

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-V H-L w -V H-L w -L red CH 4 blueC2H6C2H6 greenC3H8C3H8 magenta i-C 4 H 10 Pressure Temperature [K]

Equilibrium conditions for some other gases I-H-V H-L w - V H-L w -L H-L-V red N2N2 blueCO 2 magenta H2SH2S Pressure Temperature [K]

Influence salts, organic compounds Temparature log Pressure H 2 O + CH 4 H - L w - V Concentration H 2 O + CH 4 + NaCl of H 2 O + CH 4 + MeOH H 2 O + CH 4 + cyclic organic component

Application: desalination sea water CO 2 or air brine gas recycle Hydrate formation Separation drink H 2 O decomposition

Threat CH 4 is a much more serious contributant to greenhouse effect than CO 2. 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.