LysarkKaasa2008

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Classification: Internal Status: Draft

Glycol injection and processing

Course: TPG 4140 NATURGASS

Date: 16.10.2008

Baard Kaasa, StatoilHydro



2

Introduction – flow assurance

PipelineHigh temp Low temp

Water condensation

Low T high P

Hydrates



3

Introduction – flow assurance

Snøhvit pipelineHigh temp Low temp

MEG injection – prevents hydrates

CO2 Low pH ~4

Corrosion

NaOH/NaHCO3

High pH ~6-7

Formation water?

3

2

3

2CaCOCOCa =+

−+

SCALE



4

A typical MEG loop

Pipeline

MEG injection l ine

Lean MEG

MEG: 90 wt%Rate: 150-200 m3/d

Rich MEG

MEG: 60 wt%

Rate: 220-300 m3/d

Water

Condensed water: 70-100 m3/d

Formation water: 0-10 m3/d

Process plant withMEG regeneration

Water and waste

Rate: 70-110 m3/d

Wells



5

Introduction – MEG processing• Separation/filtration/centrifuges/settling

– MEG is viscous, slow settling, clogs filters

– Particles may be very small. (Especially corrosion products)

• Scale/plugging

– Scaling in heat exchangers (typical carbonates)

– Sedimentation – salt will settle and may plug of flow is low

• Hydrocarbons - Dissolve in rich MEG

– Hydrocarbons in reflux water

– Hydrocarbons in reflux drum and other vessels

– Extra load on vacuum system

• Corrosion

– A lot of CO2 is released – low pH in vacuum system



6

Overview• Flow assurance

– Hydrate

– Water equilibria

– Corrosion

– Scale

– Thermodynamics of MEG mixtures

• Glycol processing

– Pre treatment

– Water removal (regeneration)

– Salt removal (reclamation)

– Waste handling



7

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

– Forms at low temperature and high pressure

• Water will form hydrates if the chemical potential of water in the hydrate form is

lower than that of free water • Requirements for hydrate formation

– Free water

– Small gas molecules (N2, CO2, CH4, C2,C3,…)

– Low temperature – High pressure



8

Hydrate phase diagramHydrate equilibrium curve

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20 25

Temperature (°C)

P r e s s u

r e ( b a r )

Hydrats can form

Hydrats can NOT form

Equilbrium curve



9

Hydrate prevention• Hydrate formation reaction (simplified)

• Chemical potential

– If reaction will go

• Chemical potential of gas is mainly given by pressure and temperature and cannormally not be changed

• Chemical potential of hydrate can not be changed

• Chemical potential of water

– If we dilute water to get xH2O lower, the chemical potential of water will belower and the reaction is shiftet to the left -> no hydrates

( ) ( )hyd O H gaslO H 22

↔+

2 2

hyd l

H O H O gas

μ μ μ μ Δ = − −0μ Δ <

2 2 2 2 2 2

ln lnl

H O H O H O H O H O H O RT a RT xμ μ μ γ = + = +o o



10

Hydrate prevention – diluting the water • Hydrates can be prevented with “anything” that reduces the molefraction of

water Hydrate inhibition effect

0

10

20

30

40

50

60

0 0.1 0.2 0.3 0.4 0.5 0.6

Molefraction inhibitor

D e c r e a s e i n H y d r a

t e t e m p e r a t u r e ( C )

dT MeOH

dT Ethylene Glycol

dT Diethylene Glycol

Nielsen and Bucklin

( )72 ln 1 InhibT xΔ = − −



11

Lean MEG injection rate - Example• A pipeline transports gas and water condenses: 100 m3/d

• How much lean MEG (90 wt%) must be injected to keep MEG concentrationabove 60 wt%?

• Assumption: Density of water and MEG = 1000 kg/m3

– We want:

– Inserting lean MEG

– Rearranging

– Rich MEG volume

2

0.6 MEG

MEG H O

V

V V =

+

0.90.60.9 0.1

LM

LM Cond LM

V V V V

=+ +

( )

0.62

0.9 0.6

Cond LM Cond

V V V = =

−

Total water:Condensed water + water in lean MEG

3 RM Cond V V =

Lean MEG:

Lean in water – 90 wt%

Rich MEG:

Rich in water – 40-70 wt%



12

Water vapour pressure• Injection of MEG will “dry” the gas phase

– Addition of MEG reduces molefraction of water in the aqueous phase• Water has less tendency to go to other phases – same as for hydrate inhibition

– Question: How much is the water content of the gas reduced if MEG

concentration is 60 wt%: MwH2O=18 mole, MwMEG=62 g/mole

– Answer : Assume 1 kg, 600 gram MEG, 400 gram water

MEG: 600/62=9.68 molesH2O: 400/18=22.2 moles

Vapour pressure of water is reduced with 30%

2 2 2 2 H O tot H O H O H o y P P x P= =

o

2

2

2

22.2

0.7022.2 9.68

H O

H O

H O MEG

n

x n n= = =+ +



13

Corrosion• Long pipelines: Carbon steel

• Condensed water has no buffer capacity

– CO2 in gas -> pH=3.5 - 4.5

• After startup on Kollsnes they produced about 20 tonns of iron!

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

Corrosion



14

pH stabilization

• Increase pH in pipeline

– Add NaOH or NaHCO3 to lean MEG

– Higher pH – less corrosive – Formation of FeCO3 protective film

2 3 NaOH CO NaHCO+ =

Kollsnes pH stabilization



15

Scale

• Scale means precipitation of salt onto a surface

• Main types of scale:

– Sulphates• BaSO4, CaSO4 ,CaSO4*2H2O, SrSO4

– Carbonates• CaCO3, FeCO3

– Other

• NaCl, NaHCO3, FeS, HgS

• Scale will:

– Block pipelines and heat exchangers

– Fill separators – Reduce heat transfer in exchangers

– Valves etc. can get stuck



16

Scale formation mechanisms

• Sulphate, chloride

• Mainly dependent on: – Ion concentrations

– Temperature

• Forms typically

– When formation water and seawaterare mixed

– Formation water: Ca2+, Ba2+, Sr 2+

– Seawater: SO42-

• Carbonates, sulphides

• Mainly dependent on: – pH

– Ion concentration

– Temperature

• Forms typically

– Pressure reduction• CO2 evaporates, pH increases

– Temperature increase• Lower solubil ity, CO2 goes off

2 2

4 4 Ba SO BaSO+ −

+ = 2 2

3 3Ca CO CaCO+ −

+ =

2

2 3 3CO H HCO H CO

+ − + −

→ + → +Increasing pH



17

Saturation ratio

• Reaction:

• Equilibrium:

– Solubility product. At equilibrium, the product of the ion concentrations is aconstant

• Ksp is a funct ion of temperature and pressure (and non-ideal behaviour)

• Saturation ratio

– Tells if a salt is supersaturated and can precipitate

( )( )

Na Cl

sp

m mSR NaCl

K NaCl

+ −

=

Na Cl NaCl+ −

+ =

( )sp Na ClK NaCl m m+ −=

SR>1 Precipitation can occur SR=1 EquilibriumSR<1 Undersaturated, salt may dissolve



18

Salt solubility in MEG systems

• MEG and water mix completely at all ratios

• The mixture is then the solvent – and it has different properties than water

– Salt solubility is lower in MEG

– Many salts have a low-point at about 70-80 wt% MEG

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100

w t% MEG

S o l u b i l i t y

[ m

]

Model-NaCl

Literature-NaCl

model-KCl

Literature-KCl

0

2

4

6

8

10

12

14

16

0 0.2 0.4 0.6 0.8 1

molfract ion MEG

C a

l c i t e

S o l u b i l i t y

[ m

m

o l / K g

]

25C

60C

80C



19

Gas solubil ity in MEG

• Solubility of CO2 and CH4 in MEG. PCO2=PCH4=1 bar, T=20°C

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100

MEG (w t%)

C O 2 s o l u b i l i t y ( m m o l / k g )

0

0.5

1

1.5

2

2.5

C H 4 s o l u b i l i t y ( m m o l / k g

CO2

CH4



20

pH in the MEG solutions

• pH in MEG solutions is difficult to measure because

– The pH scale in MEG is different from water

– MEG has effect on the pH electrode – changes the measured pH

– Calibration buffers does not contain MEG• Solution

– Calibrate as normal

– Apply correction

3.5

4.0

4.5

5.0

5.5

6.0

0 20 40 60 80 100

wt % MEG

p H

Buffer-pH*

Measured-pH

pH pH pH Meas Δ+=*

True pH = Measured + Correction



21

Overview

• Flow assurance

– Hydrate

– Water equilibria

– Corrosion – Scale

– Thermodynamics of MEG mixtures

• Glycol processing

– Pre treatment

– Water removal (regeneration)

– Salt removal (reclamation)

– Waste handling



22

MEG regeneration

Rich MEG MEG regeneration Lean MEG

MEG 50-70 wt%Water

pH stabilizer SaltsCorrosion productsProduction chemicals CI, SIHydrocarbons

MEG 90 wt%

pH stabilizer Production chemicals CI, SI

Water & Waste



23

Pre treatment

• Purpose

– Stabilize MEG

– Remove HC

– Buffer between production and regen – Removal of impurities

• Filtration

• Centrifuges

• Precipitation

• Sedimentation

• Man challenges

– Scale in heat exchanger

– Separation (MEG in condensate, condensate in MEG)

Inlet separator

Degasser

Heater

Rich MEG storage

Gas

Condensate

Condensate

Gas



24

Water removal (regeneration)

Kollsnes Snøhvit

Pressure: 1.1-1.3 bar

Temperature: 140-150°C



25

Salt removal (reclamation)

Snøhvit

Pressure: 0.1-25 bar

Temperature: 110-130°C

Flash

separator

Salt freevapour

Saltfeed

Circulation pump

Heatexchanger

Vacumreciever

Water

Vacumpump

Solids removal

Lean MEG



26

Pre treatment heat exchanger



27

Reclaimer bottom section



28

Cooling water Heat exchanger



29

Sediment tanks



30

Process combinations

• Only regeneration (Kollsnes)

– Removes only water, no salt

– pH stabilizer is conserved

• Full reclamation (Åsgard B, Vega/Gjøa)

– Removes all salt and water

• Partial reclamation (Snøhvit, Ormen Lange)

– A fraction of the MEG is reclaimer

– Upstream (60wt%) or downstream (90wt%)

Salt removal

Reclamation

Water removal

Regeneration

Salt removal

Reclamation

Water removal

Regeneration

Flash

separator

Salt freevapour

Saltfeed

Circulation pump

Heatexchanger

Vacumreciever

Water

Vacumpump

Solids removal

Lean MEG



31

VLE for water-Glycol systems

• Water-Glycol behaves close to ideal – Raults law can be used (within 5%)

2 2 2 H O H O H o

MEG MEG MEG

P x P

P x P

=

=

o

o

Vapour pressure of w ater and MEG

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100 120 140 160 180 200

Temperatur e (°C)

P v a p ( b a r )

PH2O

PMEG

T-functions ln ln *a

P b c T d T T

= + + +o

Comp H2O MEG

a -7875.17 -7878.1

b 86.9198 0.143701

c -11.783 3.72897

d 0.010658 -0.0133992

32



32

Regeneration temperature

• Regeneration

– Liquid composition is known• 90wt% MEG=> xMEG=0.723

– Iterate on temperature

2 2

0.277 0.723tot H O MEG H o MEGP P P P P= + = +

o o

138

140

142

144

146

148

150

152

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Reboile r pressure (bar)

B o i l i n g t e m p e r a t u r e ( ° C

)

ln ln *

aP b c T d T

T = + + +

o

33



33

Reclamation temperature

• Reclamation

– Vapour composition is known (same as feed)• 60 wt% MEG => yMEG=0.3

• Pressure is 0.15 bara (vacuum)• What is temperature?

2

2

0.3

0.7

MEG MEG

tot

H O

H O

tot

P y

P

P yP

= =

= = ( )2 2 2 2

0.3

0.7 1

MEG tot MEG MEG

H O tot H O H O MEG H O

P P x P

P P x P x P

= =

= = = −

o

o o

2

2

0.3 0.7

MEG H O

calc

H O MEG

P PP

P P

=+

o o

o o

2

2

0.3

0.7

tot MEG

MEG

tot H O

H O

P x

P

P x

P

=

=

o

oIterate on temperatureuntil Pcalc=Ptot

=> Eliminate x

34



34

MEG reclamation P-T diagram

20

30

40

50

60

70

80

90

100

1 0 0

1 0 2

1 0 4

1 0 6

1 0 8

1 1 0

1 1 2

1 1 4

1 1 6

1 1 8

1 2 0

1 2 2

1 2 4

1 2 6

1 2 8

1 3 0

Temperature (°C)

w t % M

E G

i n F e e d a n d V a p o u

r

88

90

92

94

96

98

100

w t % M

E G

i n F l a s h S e p L i q u i d

Gas or Feed P=0.1Gas or Feed P=0.12Gas or Feed P=0.14Gas or Feed P=0.16Gas or Feed P=0.18Gas or Feed P=0.2Liq P=0.1Liq P=0.12Liq P=0.14Liq P=0.16Liq P=0.18

Liq P=0.2

Flash sep l iq

Vapour or Feed

35



35

Energy consumption - reclamation

• In a reclaimer, both MEG and water is evaporated

– Example:• Feed: 300 m3/d 60 wt% MEG

• Feed temp: 30°C, boi ling temp, 120°C

– Answer:• MEG 180 m3/d=180 000 kg/d

• H2O 120 m3/d=120 000 kg/d

– Heating from 30 to 120°C• MEG 180 000*3.10*90=5.02*107 kJ/d = 580 kW

• H2O 120 000*4.16*90=4.49*107 kJ/d = 520 kW 1 100 kW

– Vaporization• MEG 180 000*968 =1.74*108 kJ/d = 2 016 kW

• H2O 120 000*2603=3.12*108 kJ/d = 3 615 kW 5 631 kW 6.7 MW

Water MEG

Heat capacity (kJ/kg K) 4.16 3.1Heat of vaporization (kJ/kg) 2603 968

36



36

Waste handling

• Water from distillation

– Onshore: Sent to water treatment and then disposed

– Offshore: Disposed

• Salt

– Up to 1.5 tons/d is possible

– Onshore: No good solution, treated as special waste

– Offshore: Dissolved in water from distillation and then disposed

• Problem: High content of heavy and polar hydrocarbons

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