Aker Solutions Presentation - Drying of Natural Gas

Drying of natural gas Thomas Førde, October 21, 2010

Troll A © 2008 Aker Solutions

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Layout 1. Introduction/motivation 2. Industrial examples 3. Theory drying

• Dehydration 4. Summary

Slide 2

© 2008 Aker Solutions

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Background

Introduction

Explanations ■ Raw natural gas; gas produced

from the well ■ Sour natural gas; contains

hydrogen sulfide H2S or carbon dioxide CO2 ■ Sweet natural gas; contains little

sulfur and carbon dioxide ■ Rich natural gas; contains larger

quantities of higher hydrocarbons ■ Wet natural gas; is saturated with

water vapor under natural conditions

Kårstø Statoilhydro photo

Petroleum technology volume 1-2 chapter 13 natural gas Slide 3


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Introduction

Introduction Gas specifications

Gas and liquid contracts usually contain the following basic considerations: ■

Gas 1. Minimum, maximum and nominal delivery pressure 2. Maximum water content (expressed as a dewpoint at a given pressure or concentration) 3. Maximum condensable hydrocarbon content (expressed as a hydrocarbon dewpoint ) 4. Allowable concentration of contaminants (H2S, carbon disulfide…) 5. Minimum and maximum heating value 6. Cleanliness (allowable solids concentration)

■

Liquid 1. Quality of product (expressed as vapor pressure, relative or absolute density) 2. Specification (color, concentration of contaminants) 3. Maximum water content

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Introduction

Motivation Treating

■ Water must be removed

● Solid hydrates with hydrocarbons or hydrogen sulfide ● Slugs in pipeline ● Corrosive H2S and CO2 ■ Hydrogen sulfide (H2S) must be

removed ● Toxic and corrosive ● Often done centralized treatment plants ■ Nitrogen

● No heating value Petroleum technology volume 1-2 chapter 13 natural gas, Natural gas production processing transport A.Rojey et.al Slide 5


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Motivation

Introduction

Flow configurations •

Well-stream from sub-sea/platform to shore (LNG; Snøhvit, gas export; Troll and Ormen Lange)

•

Platform with full gas processing gas export (Sleipner) Off shore platform processing

Pipe line to europe

Troll, ormen lange LNG

snøhvit Refinery and petrochemicals

Pipe line

Sleipner

Troll

1: Off shore to land, pipe line demands

3: LNG composition demands

2: Export pipe line, demands

4: Condensate composition demands

Principal sketch natural gas, well to consumer Slide 6


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Introduction

Motivation Typical north sea natural gas composition

Major components (mol percentage dry gas) in some north sea gas reservoirs H20

N2

H2S

CO2

He

Methane

Ethane

Propane

Other*

TrollAA

Saturated

1.74

-

0.22

-

92.69

3.53

1.51

0.31

KristinA

Saturated

0.32

3.36

71.08

8.70

4.13

12.4

South-east asian field

Saturated

0.38

65.8

24.8

0.47

0.15

7.9

SleipnerB

Saturated

1.6

3.42

83

8.6

3

0.38

Typical [1]

Saturated

0-15

0-30 0-5

75-99

1-15

1-10

0-1

0.49

0-3

A Well stream, B Pipeline stream

It can be seen from the table, that Troll produced very lean gas. Other fields contains more CO2 and heavy components. Slide 7 1 Petroleum technology chapter 13 * hydrocarbons


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Industrial examples

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Industrial

Natural gas processing

Principal sketch natural gas processing route Slide 9


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Industrial

Industrial examples Troll, Kolsnes onshore plant

Simplified flow sheet Troll onshore gas treatment plant Kolsnes Slide 10


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Industrial

Industrial examples Principal sketch Troll, MEG* System

Background: •Troll is located in the north part of the North Sea, about 65 km west of Kolsnes • Ocean depth is above 300 meter • The field is divided into Troll east and Troll west • 2/3 of the recoverable gas reserve is located in the east * Monoethylene Glycol (MEG) also called ethylene glycol (EG) Slide 11


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Troll Dehydration system MEG (Pressure, BARG) Inlet gas separator Feed gas from slug catchers

Lean gas to pipeline compressors

(68.5) Turboexpander <-11.7> Suction drum

(78.4) <-0.7> Dewpoint separator

(89.5) <-5.1>

(90) <5>

(69.4)<-20.2>

(67) <-21>

(69)<-20.2> Condensate and Glycol

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Industrial

Principal sketch Kristin All processing offshore Q

Q

Kristin is a high pressure field (900 in the well, choke sea bottom to 350 bar) Ocean depth is about 350 meters Gas is transported to Kårstø Economic choice of technology; takes advantage of high well pressure and existing single phase pipe-line to Kårstø Full processing offshore to meet existing pipe-line spec (105 cricondenbar) inlet pipeline pressure 211 bar and 50 degrees Celsius Gas is delivered at Kårstø at 100 bar Slide 13


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Kristin Liquid separation system To Dehydration system

<26> 3st stage (25) recompressor Inlet wet gas

Inlet separator

<30> (7) 2st stage recompressor

<112> (87)

<30> (1.7) 1st stage recompressor

<120> (26) 2nd stage separator

(Pressure, BarA)

Pres s incre ure asing

<74> (2.15) 3rd stage separator

pre red ssu uc re tio n

To condensate storage

Sketch of Kristin’s liquid separation system Slide 14


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Kristin Separation re-compressor package

From separator

Out of recompressor

Compressor separator

To separator

Sketch of Kristin’s separator recompression system Slide 15


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Industrial

Principal sketch Kristin All processing offshore Q

Q

Kristin is a high pressure field (900 in the well, choke sea bottom to 350 bar) Ocean depth is about 350 meters Gas is transported to Kårstø Economic choice of technology; takes advantage of high well pressure and existing single phase pipe-line to Kårstø Full processing offshore to meet existing pipe-line spec (105 cricondenbar) inlet pipeline pressure 211 bar and 50 degrees Celsius Gas is delivered at Kårstø at 100 bar Slide 16


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Kristin De-hydration (TEG) system

Sketch of Kristin’s dehydration system TEG: Triethylene glycol

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Industrial

Snøhvit Principal sketch CO2

Feed from pipeline

To pipeline

Slug catcher

MEG Recovery

Inlet separation

CO2 Removal

Dehydration

Mercury Removal

Condensate treatment

Natural gas liquefaction

LNG storage

Condensate storage

Fractionation

LPG storage

First developed field in the Barents sea Ocean depth of 300-350 meters A gas field with condensate and an underlying thin oil zone Choice of technology: Make LNG, no existing gas lines to Europe Slide 18


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Snøhvit dehydration system Molecular sieve Regeneration gas

(63.0)<230>

Dry gas (64.0) <27.6 >

Example of Molecular sieves

(pressure, barA)

Hot Oil Regeneration gas (63.7) <27.5 >

Slide 19

(64.9) <26.6 > Wet gas (63.2) <233.0 >

Snøhvit’s molecular sieve © 2008 Aker Solutions

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Summary Introduction, industrial examples and pipeline

These points have been discussed/explained: ■ General facts about natural gas ■ The dehydration system at:

● Troll (onshore), MEG injection and dehydration by cooling (turboexpanders) ● Kristin (offshore), dehydration by absorption (TEG system) ● Snøhvit (onshore), dehydration by adsorption (molsieve) ■ Some of the issues related to transport of natural gas in pipelines

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Dehydration

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Dehydration

Natural gas processing

Principal sketch of a natural gas processing plant Slide 22


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Dehydration

Dehydration

Dehydration is the process of removing water from a gas and/or liquid Natural gas is commercially dehydrated in one of three ways 1.

Absorption (Glycol dehydration)

2.

Adsorption (Mol sieve, silica gel, or activated

alumina)

3.

Condensation (cooling) (Refrigeration with glycol or methanol injection)

Four glycols are used for dehydration and/or inhibition 1.

Monoethylene Glycol (MEG) also called ethylene glycol (EG)

2.

Diethylene glycol (DEG)

3.

Triethylene glycol (TEG)

4.

Tetraetylhene glycol (T4EG)

• Absorption and refrigeration with hydrate inhibition is the most common dehydration process used to meet pipeline sales specifications • Adsorption processes are used to obtain very low water contents required in low temperature processes, for example LNG • TEG is most common in absorption systems • MEG is most common in glycol injection systems Slide 23


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Absorption Dehydration

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Natural gas is dried by absorption, often in a countercurrent scrubbing unit A liquid having a strong affinity for water is used as an absorbent A good absorbent should have: 1. Strong affinity for water 2. Low cost 3. Non corrosive 4. Low affinity for hydrocarbons and 5. 6. 7. 8. 9.

acid gases Thermal stability Easy regeneration Low viscosity Low vapor pressure at the contact temperature Low tendency to foam Slide 25

Increasing values Molecular weight

MEG

DEG

TEG

T4EG

62 – 194

Viscosity (25 C)

MEG

DEG

TEG

T4EG

17- 49

Freezing point C

MEG

T4EG

DEG

TEG

-13 - -7

Vapor pressure 25 C

MEG

DEG

TEG

T4EG

Basic glycol properties


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Basic glycol dehydration unit

Simplified flow diagram for a glycol dehydration unit. from the GPSA Engineering Data Book, 11th ed. Slide 26


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The glycol dehydration unit Wet gas (no liquid water) enter bottom of absorber and flows countercurrent to the glycol. Lean glycol enters at the top One, two pass trays

Reactor

■

Absorber internal ● Tray ■ Bubble cap ■ Valve ■ Sieve

Maximize Contact area and time Gas/glycol

● Packing ■ Berl Saddle, Raschig Ring…… Valve tray

Bubble Cap

Bearl Saddle Sieve tray

Bubble Cap tray Slide 27


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Absorber design Mol fraction water in gas

Design parameters ■ Purity demand ■ Working temperatures ■ Working pressure ■  Choice of absorbent

Design procedure ■ Mass balance circulate enough glycol to ■ ■ ■

■

■

absorb the water in the gas Gas rate tank diameter (flooding) Equilibrium analysis number of equilibrium stages Real analysis, have to take into account the reaction kinetic and contact time between glycol and gas. Gives number of actual trays Dryer glycol higher concentration differences  higher reaction kinetic higher efficiency  more expensive and heavier glycol regeneration system Higher glycol circulation rate higher concentration differences higher reaction kinetic higher efficiency higher pressure drop  more expensive and heavier pumps Slide 28

Yb w flo s Ga Y mol frac. Water Top of tower Bottom of gas phase e tower lin P Yt Yb* O e n i l EQ Y* EQ mol frac. w o l f Water gas phase ol Yt* Glyc

Mol fraction water in glycol Principal sketch assuming: • Mass transfer are controlled by resistance on the gas side • Straight operation and equilibrium lines of mol fraction water in the gas phase

No. of EQ stages  No. of actual stages


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Glycol dehydration unit Working principle

Typical profiles of the mol fraction of water in glycol as a function of tower height. For tray and structural packing

Typical profiles of the mol fraction of water in gas as a function of tower height. For tray and structural packing

• Minimum tray spacing 610 mm

• Discrete and continues concentration profile

• Flooding, foaming

• Equilibrium assumption

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Glycol regeneration Alternatives

Cool

Increased temperature

A; Wet stripping gas B; stripping gas still column

Water

Re boiler

Rich TEG

Heat Exchanger TEG unit

Heat

A; Stripping gas

A) Open stripping loop B) Closed stripping loop

C) Cold finger

A) Any inert gas is suitable. Theoretically best to insert stripping gas between re boiler and surge tank B) A closed stripping loop, isooctane can be used. Vaporizes at re-boiler temperature and condenses and can be separated from water in a three phase separator. High stripping gas rates with little venting of hydrocarbons. Glycol cons> 99.99% (w/w) has been achieved. Slide 30

C) A cold finger is inserted into a bucket in the surge drum vapor space. A TEG mixture rich in water condenses out. This mixture is taped off. H2O partial pressure is lowered and lean glycol concentration is increased. 99.5-99.9 % (w/w) glycol has been achieved.


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Glycol regeneration Component Reboiler: Temperature should not exceed 204 C (TEG) due to degradation. Some degradation of glycol in contact with heat transfer surface  maximum heat flux rates. Heat provided with direct fired fire tubes immersed in the bath, hot oil, steam or electric resistance heating. Stripping Colum:

Flash tank: Used to remove light hydrocarbons, CO2, H2S. Operation pressure 15% of the contactor operating pressure. Filters: Captures chemical impurities and solid particles. Pressure drop is measured and used as a replacing criteria. Slide 31

Can be trayed or structural packed. Stripping gas lowers the partial pressure of H2O in the gas phase, and more water can be absorbed by the gas (Raoults law). Surge drum: Retention time >20 min Be able to hold all the re-boiler glycol, to allow repair or inspection of the re-boiler heating coil.


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Glycol absorption Pros and cons

Pros

Cons

■

Low initial cost

■

Low pressure drop across absorption towers

■

Recharging of towers present no problems

■

Materials that would cause fouling of some solid adsorbents can be tolerated in the contactor

Slide 32

■

Suspended matter, such as dirt, scale, and iron oxide may contaminate glycol solutions

■

Overheating of solutions may produce both low and high boiling decomposition products

■

The resultant sludge may collect on heating surfaces, causing some loss in efficiency, or, in severe cases, complete flow stoppage

■

When both oxygen and hydrogen sulfide is present, corrosion may become a problem because of the formation of acid material in the glycol solution

■

Liquids such as water, light hydrocarbons or lubrication oils in inlet gas may require installation of an efficient separator ahead of the absorber. Highly mineralized water entering the system with inlet gas may, over long periods crystallize and fill the re-boiler with solid salts

■

Foaming of solution may occur with a resultant carry-over of liquid. The addition of a small quantity of antifoam compound usually remedies this problem


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Dehydration by cooling

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Dehydration by cooling NGL recovery

Refrigeration system

A refrigeration system lowers the temperature of a fluid or gas below that possible when using air or water at ambient conditions. ■ Refrigeration systems are used for

● Removing of water

● Chilling natural gas for NGL extraction ● Chilling natural gas for hydrocarbon dew-point control ● LPG product storage ● Natural gas liquefaction (LNG) ■

Refrigeration processes: ● Mechanical refrigeration ■ Compression (uses energy in form of work to pump heat)

■ Absorption (use energy in form of heat to pump heat, ammonia systems)

● Expansion refrigeration ■ Valve expansion (Joule Thompson) ■ Turbine expansion (Turbo expander)

Slide 34

Natural gas liquid fractions as a function of temperature at atmospheric pressure


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Refrigeration cycle Principal thermodynamic path

Liquid recovery by refrigeration

Thermodynamic path A-B,E cooled by heat exchange with the process gas.

B-C Natural gas is cooled by heat exchange with the refrigeration cycle. The gas temperature is lowered at constant pressure. E-F’ Natural gas is cooled by isentropic (constant entropy S) expansion through a turbine (turbo expander), EF actual path. B-D Natural gas is cooled by isenthalpic (constant enthalpy) expansion through a valve (Joule Thompson). Slide 35


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Principal sketch of a refrigeration cycle

Natural gas

Refrigeration is achieved by vaporization at relatively low refrigerant pressure. The refrigerant can be a propane or sometimes a halogen of the Freon type. Slide 36


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Turbo expander cycle (Troll gas)

1 Feed gas

110

Path turbo expander Feed gas phase envelope Path joule thompson

1-2 Gas-gas heat exchanger 2-3 Suction drum

90 Pressure [Bar]

3-4 Turbine expander 70

4-5 Dewpoint separator 5-6 Gas-gas heat exchanger

50

6-7 Compression 30

A hydrate inhibitor (MEG) is often injected upstream of the heat exchanger, if the gas is unhydrated

10 -170

-140

-110

-80

-50

-20

-10

10

40

Temperature [C]

Dehydrated gas

1

Lean gas to pipeline compressors

Turboexpander Suction drum

6

7

3 2

Phase envelope based Troll, dehydrated gas

Dewpoint separator

4

5

Turbo expander process for NGL extraction

Condensate and Glycol Slide 37


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Joule Thompson cycle (Troll gas)

1 Feed gas

110

Path turbo expander Feed gas phase envelope Path joule thompson

1-2 Gas-gas heat exchanger

90 Pressure [Bar]

2-3 Suction drum 70

3-4 Valve expander 4-5 Dewpoint separator

50

5-6 Gas-gas heat exchanger

30

A hydrate inhibitor (MEG) is often injected upstream of the heat exchanger, if the gas is unhydrated.

10 -170

-140

-110

-80

-50

-20

-10

10

40

Temperature [C] Lean gas to pipeline compressors

Turboexpander Suction drum

6

3

Inlet gas

1

2

Phase envelope based on Troll, dehydrated gas

Dewpoint separator

4

5

Joule Thompson process for NGL extraction

(69)<-20.2> Condensate and Glycol Slide 38


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Dehydration by adsorption

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Dehydration by sorption


Adsorption describes any process where gas molecules are held on the surface of a solid by surface forces. Adsorbents may be divided into two classes. Species is adsorbed due to physisorption and capillary condensation ● Species is adsorbed due to chemisorption (not much used in natural gas processing) ●

A sorbent must have the following properties: 1. 2. 3. 4. 5. 6. 7.

High adsorption capacity at equilibrium Large surface area Easily and economically regenerated Fast adsorption kinetics Low pressure drop High cyclic stability (kinetic and capacity) No significant volume change (swelling shrinking)

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The commercial available sorbents can be divided into three broad categories: 1. Gel A granular amorphous solid (silica gel (SiO2), alumina gel Al2O3) 2. Alumina

Hydrated form of aluminum oxide Al2O3, activated by drying off part of the hydrated water adsorbed on the surface

3. Molecular sieves

Alkali metal crystalline aluminosilicates, very similar to natural clays

Example of sorbents: ■

Silica gel (Gel type) Outlet gas water content down to 10 ppm (v/v) and dew point -60 C can be achieved Regenerated between 120 and 200 C It adsorbs hydrocarbons, which are desorbed during regeneration Silica gel is destroyed by free water which causes the granules to burst, and react with bases

■

Activated alumina Al2O3

Outlet gas water content <1 ppm (v/v), outlet dew point -73 C can be achieved Heavy hydrocarbons are adsobed but can not be desorbed during regeneration

■

Molecular sieves (zeolites) Outlet gas water content down to 0.03 ppm (v/v) , outlet dew point -100 C Water is adsorbed in a micro porous structure The presence of carbonyl sulfide (COS) and carbon disulfide (CS2) should be avoided The adsorbent must be replaced frequently (about every three year) The water content in the feed must be low Slide 41


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Dehydration By sorption

Principal sketch Adsorbent system

Regeneration gas

Operation

Regeneration

Process gas

Molecular sieves Regeneration gas

Process gas

Flow sheet of a basic two tower adsorption system with regeneration

http://www.uop.com/objects/96%20MolecularSieves.pdf Slide 42


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Adsorption

Dry gas

Concentration profiles Active Zone Mass transfer Zone Equilibrium Zone

Wet gas

Variation of adsorption zones with time and height

Schematic view of reactor bed with adsorption zones

• Equilibrium zone: Sorbent is saturated with water. • Mass transfer zone: All the mass transfer takes place in this zone. • Active zone: The sorbent has its full capacity for water, contains only residual water left from regeneration cycle. Slide 43


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Adsorption General point and re-generation

Design parameters ■ Number of adsorption units regeneration time ■ Gas velocity and allowable pressure drop  diameter ■ Good internal flow distribution avoid channeling ■ Proper pre-treating of the gas ● Degradation due to loss of effective surface area Principal sketch of reactor temperature during ● Degradation due to blockage of regeneration small capillary or lattice T0-TA heating of the reactor openings Proper heat loss management (insulation internal/external) optimize regeneration ■ Proper heat recovery ■ Possible to replace adsorbent ■

TA-TB evaporation and breaking of surface forces TB-TC removing of heavy contaminants and residual water TC Cooling, heat recovery phase

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Summary dehydration Different dehydration technologies have been discussed ● Absorption ■

Glycol system » Trayed towers » Structural packing

● ● ●

Concentration profiles Design guide lines System components

● Cooling ■

System ● Compressor cooling ● Turbo expander ● Joule Thompson

● Adsorption ■ ■ ■

Concentration profiles Design guide lines System component/operation

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CO2 capture technology

Slide 46


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CO2 capture from energy related sources

Fossil fuel Air

Combustion

Flue gas

CO2 separation

Energy

N2 ,O2

CO2 Air

Fossil fuel

Gasification/ reforming Air/O2 Steam

Energy

H2, CO2

CO2 separation

H2

Combustion

CO2

N2 ,O2 , H2O

Energy

CO2 capture from large scale power plants is yet to be implemented Slide 47


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Overview CO2 capture technologies* Separation task

Capture technologies

Solvents (Absorption)

Membranes

Solid sorbents

Cryogenic

Process Streams

Postcombustion capture

Oxyfuel Combustion capture

Pre-Combustion Capture

CO2/CH4

CO2/N2

N2/O2

CO2/H2

Current

Physical solvents Chemical Solvents

Polymeric

Emerging Improved solvents Novel contacting equipment Improved design of processes Ceramic Facilitated transport Carbon Contactors

Current

Emerging

Chemical solvents

Improved solvents Novel contacting equipment Improved design of processes

Polymeric

Current

n. a.

Emerging

Biomimetic solvents, e.g. hemoglobinederivatives

Physical solvent Chemical solvents

Novel contacting equipment Improved design of processes

Ion transport membranes

Ceramic Facilitated transport Carbon Contactors

Polymeric

Polymeric Facilitated transport

Zeolites

Carbonates

Zeolites

Activated carbon

Activated carbon

Carbon based sorbents

Activated carbon

Liquefaction

Current

Improved chemical solvents

Zeolites

RyanHolmes process

Emerging

Hybrid processes

Distillation

Adsorbents for O2/N2 separation Perovskites Oxygen chemical looping

Activated carbon

Improved distillation

Liquefaction

Zeolites

Alumina

Ceramic Palladium Reactors Contactors

Carbonates Hydrotalcites Silicates

Hybrid processes

* From IPCC special report on Carbon Dioxide Capture and Storage, 2005 Slide 48


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Selcetion of CO2 capture technology

http://www.uop.com/gasprocessing/6010.html

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Typical CO2 absorption loop

Product gas

Product Gas KO Drum

Acid gas Water Make Up Water Wash Pumps

Feedgas Feed Gas KO Drum

Acid Gas Condenser (CW)

Regen. Reflux Drum

Amine Absorber HP Lean Pump

Carbon Filter (Lean Sol) Lean Sol. Cooler (CW)

Reflux Pump

Flash gas Amine Regenerator

Rich Solvent Flash Drum

Lean-Rich Exchanger LP Lean Pump

Slide 50


Regen. Reboiler (LPS)

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Summary of presentation These points have been discussed/explained: ■

General facts about natural gas

■

Industrial dehydration examples

■

The different mechanism in gas/liquid separation

■

Different dehydration technologies ● Absorption ● Cooling ● Adorption

■ Sour gas removal

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Aker Solutions Presentation - Drying of Natural Gas

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