Hydrates and Glycols - NTNU

Hydrates and Glycols 

MEG (Mono Ethylene glycol) Injection and Processing 

NTNU september 9, 2010 

Kristian Sandengen 

Classification: Internal 2010-08-25

Agenda 

•General Introduction 

•“chemical” challenges for long multiphase flowlines 

–Hydrates 

–Corrosion 

–Scale 

•How do we battle these problems? 

•Use of Glycol (MEG) as hydrate inhibitor 

–What do we mean by a MEG loop? 

–MEG regeneration 

• Note: MEG is a common antifreeze agent, used e.g. in cars. 

Classification: Internal 2010-08-25 

MEG= Mono Ethylene Glycol

Midgard/Mikkel Tie-in on Åsgard 


Why are we using MEG? 

Low temp, high pressure and water???? 

Colder: Water condense – Multiphase flow 

Reservoir gas contains water 

dissolved as H 2 

O molecules 

in the gas 

MEG 


MEG= Mono Ethylene Glycol

Water condensation 

•Water in gas 

–Water condense at low T 

–Water evaporate at low P 

90 

80 

1.4 

1.2 

•At 5°C 

–95-99% of water has 

condensed 

Condensed water volume (m3/d) 

70 

60 

50 

40 

30 

20 

10 

Water in gas (mol%) 

Condensed water volume 

1 

0.8 

0.6 

0.4 

0.2 

Water in gas (mol%) 

Vega example: 

Gas 7 MSm 3 /d 

FW 10 m 3 /d 

P 150-85 bar 

T 110-5 °C 

0 

120 

100 

80 60 40 

Flowline temperature (°C) 

20 

0 

0 


Hydrates 

•Hydrate is water in a solid structure with small gas molecules in cavities 

–Much like snow/ice 

• Water freeze at 0ºC, what about hydrates? 

•Requirements for hydrate formation 

–Free water 

–Small gas molecules (N 2 , CO 2 , CH 4 , C 2 ,C 3 ,…) 

–Low temperature 

–High pressure 


Hydrate phase diagram 

Hydrate equilibrium curve 

Water Gas Hydrate 

200 

180 

160 

140 

Hydrats can form 

Pressure (bar) 

120 

100 

80 

Equilbrium curve 

60 

40 

20 

0 

Hydrats can NOT form 

0 5 10 15 20 25 

Temperature (°C) 


Water in pipeline - problems 

•Water condensates 

•Low T -High P -Free water 

=> Hydrate formation 

Well 

P High 

T High 

Pipeline 

P High 

T Low 



200 

180 

Hydrate prevention 


160 

140 

120 

100 

80 



60 

•How to avoid hydrate formation 

–Keep temperature high 

–Keep pressure low 

–Dilute water 

40 

20 


0 

0 5 10 15 20 25 



•Dilute water means separating water molecules and make it more difficult 

for them to react. 



•How to avoid hydrate formation 

–Keep temperature high 

–Keep pressure low 

–Dilute water 


•Dilute water means separating water molecules and make it more difficult 

for them to react. 

•We need to add something: 

–Salts works 

–Alcohol works (example methanol) 

–Glycol works (example MEG and TEG) 



•Hydrate formation reaction (simplified) 

•Chemical potential 

–If 

0 

reaction will go 

l gas H O hyd 

 

H 

2O 

 

2 

 

hyd l 

HO HO gas 

2 2 

•Chemical potential of gas is mainly given by pressure and temperature 

and can normally not be changed 

•Chemical potential of hydrate can not be changed 

•Chemical potential of water 

l 

 

 

RT ln a RT ln x 

H O H O H O H O H O H O 

2 2 2 2 2 2 

–If we dilute water to get x H2O lower, the chemical potential of water will be 

lower and the reaction is shifted to the left -> no hydrates 


Hydrate prevention – diluting the water 

•Hydrates can be prevented with “anything” that reduces the molefraction 

of water 

60 

Hydrate inhibition effect 

Decrease in Hydrate temperature (C) 

50 

40 

30 

20 

10 

dT MeOH 

dT Ethylene Glycol 

dT Diethylene Glycol 

Nielsen and Bucklin 

Alcohols 

Salt 

0 

0 0.1 0.2 0.3 0.4 0.5 0.6 

Molefraction inhibitor 

T 

72ln 1x 

 

Inhib 

 


Thermodynamic Hydrate inhibitors 

•MEG (Mono Ethylene Glycol) 

inhibits hydrate formation 

–Freezing point lowered 

Pressure [bar] 

450 

400 

350 

300 

250 

200 

150 

100 

50 

Hydrates 

M E G 

No inhib 

20% MEG 

40% MEG 

No hydrates 

0 

0 5 10 15 20 25 30 

Temperature [C] 

Inhibitor 

Water 

MeOH 

MEG 

TEG 

Density 

1.0 

0.79 

1.11 

1.12 

Viscosity (cp-25 o C) 

0.89 

0.5 

18 

36 

Freezing Point 

0 o C 

-94 o C 

-13 o C 

-5 o C 

Boiling Point 

100 o C 

65 o C 

198 o C 

278 o C 


Type of inhibitor MeOH vs. MEG 

Fire/explosion hazard 

Health hazard 

Environmental impact 

Regeneration 

Salt solubility (scale) 

Quality 

Pumping (friction loss) 

Efficiency as hydrate inhibitor 

(remember that on a mole basis all inhibitors are ~equal) 

Loss of chemical to gas phase 

MeOH 

+ 

+ 

(more per Kg or L) 

? 

MEG 

+ 

+ 

+(?) 

+ 

? 

+ 


Water in pipeline - problems 

•Water condensates 

•Low T - High P - Free water 

=> Hydrate formation 

•CO 2 

dissolved in condensed water 

CO 2 is a weak acid 

pH very low => Corrosion 

Typical pH~4 

Well 

P High 

T High 

Pipeline 

P High 

T Low 


Corrosion inhibitor 

•For corrosion to occur, there has to be water in contact with the steel 

–Long pipelines = large quantities 

–Corrosion inhibitor (CI) often environmentally unfriendly 

Absorbing end 

Inhibitor 

Hydrophobic end 

H2O 

H2O 

H2O 


Corrosion protection - “pH Stabilization” 

Initial FeCO 3 

precipitation 

Fe 

2 

 

CO 

2 

3 

 

FeCO 

3 ( 

solid) 

Protective 

FeCO 3 

film 


Corrosion 

Corrosion 

•Long pipelines: Carbon steel 

•Condensed water has no buffer capacity 

–CO 2 in gas -> pH=3.5 - 4.5 

•After start-up on Kollsnes they produced about 20 tons of iron! 

–Plugged: Heat exchangers, separators, filters, centrifuges etc. 


pH stabilization 

Fe 

2 

CO 

2 

3 

FeCO 

3 ( 

solid) 

•Increase pH in pipeline 

– Alkaline addition (NaOH) to leanMEG to 

form CO 3 

2- 

– Higher pH – less corrosive 

– Formation of FeCO 3 

protective film 

 

Kollsnes pH stabilization 

Decrease in iron concentration 


What is scale? 

Scale is precipitation of inorganic 

salts (minerals) in production 

equipment. 


Formation water – scale problem 

MEG/MeOH + Na 2 CO 3 

Scale 

P High 

T High 

Pipeline 

P High 

T Low 

Well 

Formation 

water 

•MEG/MeOH reduce salt solubility 

•Mixing point! 

– Ca 2+ from reservoir : Ca 2+ + CO 3 

2- 

= CaCO 3 

– Reacts with carbonate in the exact same manner as the (wanted) reaction with iron (Fe 2+ ) 



Fe 

2 

CO 

2 

3 

FeCO 

3 ( 

solid) 



form CO 3 

2- 

– Higher pH – less corrosive 

– Formation of FeCO 3 

protective film 

 


Decrease in iron concentration 


Scale 


Fe 

2 

2 

Ca 

CO 

2 

3 

 

CaCO 

3 ( 

solid) 




form CO 3 

2- 

Calcium comes from the formation water 


A typical MEG loop 

Pipeline 

Rich MEG 

MEG: 60 wt% 

Rate: 220-300 m 3 /d 

Process plant with 

MEG regeneration 

Water and waste 

Rate: 70-110 m 3 /d 

Wells 

Water 

Condensed water: 70-100 m 3 /d 

Formation water: 0-10 m 3 /d 

MEG injection line 

Lean MEG 

MEG: 90 wt% 

Rate: 150-200 m 3 /d 


Thermodynamics 

•Mono Ethylene Glycol (MEG) 

–MEG has a higher boiling point than water! 

–By heating one can distil water-MEG mixtures 

• Water prefers gas phase 

• MEG prefers liquid phase 



Rich MEG 

MEG 50-70 wt% 

(MEG + Water) 


Lean MEG 

MEG 90 wt% 

Water 


Water removal (regeneration) 

Kollsnes 

Pressure: 1.1-1.3 bar 

Temperature: 140-150°C 

Snøhvit 


Cooling water 

Heat exchanger 


Recycle heater 


Energy consumption 

•Water must be evaporated! 

Water 

MEG 

–Example: 

• Feed: 300 m 3 /d 60 wt% MEG 

• Feed temp: 30°C, boiling temp, 120°C 

Heat capacity (kJ/ kg K) 4.16 3.1 

Heat of vaporization (kJ/kg) 2603 968 

–Answer: 

• MEG 

• H 2 O 

180 m 3 /d=180 000 kg/d 

120 m 3 /d=120 000 kg/d 

–Heating from 30 to 120°C 

• MEG 180 000*3.10*90=5.02*10 7 kJ/d = 580 kW 

• H 2 O 120 000*4.16*90=4.49*10 7 kJ/d = 520 kW 1 100 kW 

–Vaporization 

• Not all water is evaporated (only to get 90wt% MEG) 

• H 2 O 90 000*2603=3.12*10 8 kJ/d = 2711 kW 2 700 kW 3.8 MW 



Rich MEG 

MEG 50-70 wt% 

(MEG + Water) 


Lean MEG 

MEG 90 wt% 

Water 



Rich MEG 


Lean MEG 

MEG 50-70 wt% 

Water 

pH stabilizer 

Salts 

Corrosion products 

other production chemicals 

Hydrocarbons 

MEG 90 wt% 

Water & Waste 


Salt removal (reclamation) 

Vacum 

pump 

•Need to remove 

something! 

–Dissolved 

salt/chemicals etc. 

–Removal 

• Up concentrate 

until “pollutants” 

solidify 

• Remove solid 

material 

–Evaporate all 

water+MEG 

Salt 

feed 

Flash 

separator 

Snøhvit 

Salt free 

vapour 

Circulation pump 

Heat 

exchanger 

Lean MEG 

Vacum 

reciever 

Water 

Pressure: 0.1-25 bar 

Temperature: 110-130°C 

Solids removal 


Energy consumption - reclamation 

•In a reclaimer, both MEG and water is evaporated 

–Example: 

• Feed: 300 m 3 /d 60 wt% MEG 

• Feed temp: 30°C, boiling temp, 120°C 

Water MEG 

Heat capacity (kJ/kg K) 4.16 3.1 

Heat of vaporization (kJ/kg) 2603 968 

–Answer: 

• MEG 

• H 2 O 

180 m 3 /d=180 000 kg/d 

120 m 3 /d=120 000 kg/d 

–Heating from 30 to 120°C 

• MEG 180 000*3.10*90=5.02*10 7 kJ/d = 580 kW 

• H 2 O 120 000*4.16*90=4.49*10 7 kJ/d = 520 kW 1 100 kW 

–Vaporization 

• MEG 180 000*968 =1.74*10 8 kJ/d = 2 016 kW 

• H 2 O 120 000*2603=3.12*10 8 kJ/d = 3 615 kW 5 631 kW 6.7 MW 


MEG recovery 

•MEG regeneration 

–Only water is evaporated 

–MEG passes through the 

facility as liquid phase 

–Any salts, chemicals etc. 

remains in the MEG 

–i.e. MEG loop can be 

”polluted” 

•MEG reclamation 

–Both water and MEG is 

evaporated 

–Salts, chemicals etc. are left 

behind in the liquid phase 

–i.e. salts/chemicals can be 

removed 


Solids removal 

•Sediment Tanks 

–Onshore 

•Centrifuges 

–Offshore 



200 

What have we learned? 

HYDRATES 


180 

160 

140 

120 

100 

80 



40 

•Hydrates 

20 

–An ice/snow like solid 

0 

–Forms at low temperature and high pressure 

• Need water (liquid or solid) + small molecules (e.g. CH 4 , C 2 H 6 ) 

–Reversible reaction 

• i.e. can be removed by de-pressurization or heating 

•Hydrate formation can be inhibited by: 

–Anything that dilutes water i.e. is mixable with water 

• Salt 

• Alcohol 

–Works because water molecules are separated 

60 


0 5 10 15 20 25 


•MEG 

–Denotes Mono Ethylene Glycol 

• Is an alcohol 

–Used as antifreeze agent in cars 

–Much used as hydrate inhibitor in oil industry 

• Methanol and ethanol are other much used chemicals 



CHEMISTRY 

•Corrosion 

–A problem in long carbon steel pipelines 

• Water from condensation + acidic environment due to CO 2 

–Can be avoided by addition of an alkaline chemical 

• E.g. NaOH (caustic soda) 

• Forms a protective, self-healing, film of FeCO 3 

2 

2 

Fe CO3 

FeCO solid) 

3( 

Liquid 

FeCO 3 

Metal 

•Scale 

–Unwanted formation of solids 

–Exact same mechanism as the wanted FeCO 3 protective film 

–Forms du to mixing of formation water (Ca 2+ ) with the added chemical (CO 3 

2- 

) 

Ca 

2 

CO 

2 

3 

 

CaCO 

3 ( 

solid) 


Clogged choke


MEG Loop 

•MEG Loop 

– Continuous “circle” 

• MEG injected at templates 

• Recovered at facility 

– Lean MEG: 

• “lean in water” typically 90% MEG 

– Rich MEG: 

• “Rich in water” typically 60% MEG 

•MEG Recovery 

– Recovered by distillation 

• Evaporation demands a lot of energy 

– Regeneration; Only water is removed 

• MEG loop will be “polluted” over time 

– Reclamation; water and MEG evaporated 

• Salt etc. can be removed 

• Can control “pollution” of MEG loop 

– Expected amount of formation water dictates which 

type of facility (or combination) that is needed

Hydrates and Glycols - NTNU

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