Post Combustion and Absorption Processes - US...

October 5. 2005Summer School, Hallvard F. Svendsen1

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Post Combustion and Absorption Processes

Hallvard F. SvendsenDepartment of Chemical Engineering, NTNU


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Contents

• Introduction• Technologies for post combustion CO2 removal• CO2 removal by absorption

• Hydraulic considerations• Modelling

• Flow model• Mass transfer model• Equilibrium model• Kinetics• Transport properties and thermodynamic models

• Membrane absorber


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CO2 capture technologies• Selection of process depends on various parameters such as

CO2 concentration, pressure and temperature of feed and other impurities present in the feed.


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Technologies for post combustion CO2 removal :

• Absorption in chemical and physical solvents (Alkanolamines, Rectisol, Selexol)

• Adsorption

• Membrane processes

• Cryogenic processes


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AdsorptionProblems with classical adsorbents•Low temperatures•Water vapour sensitive•Low capacity

New method suggested as a blend of absorption and adsorption

System

K2CO3 + CO2 + H2O = 2 KHCO3Oper. temperature: 100-1500C, •Needs water vapour•Reasonable capacity•Inorganic ab(ad)sorbent•Can work both in temperature and pressure swing mode


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CO2 removal by absorption

Problems associated with CO2 removal from exhaust gas by absorption

• High financial costs

• Significant energy requirements

• Production of chemical waste

Added cost per kWh produced


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Typical numbers for a 400 MW NG fired power plant:

• Produced CO2 : 1.2⋅106 t /år• Exhaust : 2⋅106 Nm3/h • Tower cross sectional area: 125-150 m2

• Efficiency loss 7-9 % points


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Problems associated with conventional absorptionplants for exhaust from NG fired power stations :

Energy consumption :• 1.7-2.3 tons steam/ton CO2 removedor• 4000-4500 MJ/ton CO2 removedDistribution of energy consumption:• Pressure increase to drive gas through process 5-8%• Pump energy for solvent recycle 2-3%• Rich amine heating 10-30%• Heat of reaction 30-40%• Stripping steam and vaporization of solvent 30-40%


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Regeneration Energy Requirement

( )( )

2 2

2

2

, ,

*,

satH O Top Des H O freebasis vap

strip H OCO Top Des Rich

P T xQ H

P T α= ∆

( )sensrich lean Am

Cp TQC

ρα α

∆=

−

2des absCOQ H= ∆

Figure 13. Energy consumption for absorption-desorption process


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Waste:From a 400MW plant comes about 1000 tons/year (special waste)Loss of solvent:

– In clean gas– In produced CO2– Oxidation and thermal degradation in the

high temperature parts of the process

Economy:Relative importance (coal fired plant):

– Energy for regeneration 67%– Energy for tower pressure drop 5%– Circulation pump 3%– Chemicals make-up 15%


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General guidelines for selecting acid gas capture technologies

0.01

0.1

1

10

100

0.0001 0.001 0.01 0.1 1 10Outlet Concentration %

Inle

t Con

cent

ratio

n %

Adsorption-Moleculer Sieves

Amines, Moleculer Sieves

Amines, Mixed solvents

Physical solvents, Mixed solvents, Amines

Membranes followed by amines Membranes

Physical solvents, Mixed solventsMembranes, Physical solvents

Physical solventsaq. Carbonate

Amines, Mixed solutions, Physical solvents,aq. Carbonates

Equal inlet and outlet


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Absorption processesRegenerativeUses temperature or pressure (or both)

for regeneration

Typical flowsheet (exhaust gas):


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Example of NG absorption process (Sleipner)

AbsorberDesorberLP-flash

HP-flash

X-VVX

Turbine

Condenser

Heater

Reboiler

Raw gas in

Clean gas out

HC out gas in

Semi-lean amine

Rich amine Lean amine


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Typical split stream configurations

Sing

le st

age

split

stre

amTw

o st

ages

split

stre

am


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Characteristics– Must cool the gas to 40-60oC– CO2- pressure at regenerator outlet is low, typically 1.6-1.8

bar– Can utilize both physical and chemical solvents

Typical equilibrium curves for physical and chemical solvents.


PCO2Pa

CCO2 mol/l

Physical

Chemical


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Commercial processes based on physical solvents

Process Name Solvent Licensor Chemical formula

Fluor Solvent Propylene carbonate Fluor Daniel C4H6O3

CH3OH

C12H27O4P

CH3C2nH4nO(n+1)CH3

C5H9NO

Rectisol Methanol (-25 oC) Lurgi-Linde AG

Estasolvan Tributyl phosphate Uhde-IFP

Selexol DMPEG Union Carbide

Purisol N-methyl-2-pyrrolidone Lurgi


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Fluor Solvent Process:

• Fluor solvent process licensed by Fluor Daniel in 1960. • Mainly used for bulk removal of CO2 from NG, NH3, H2 synthesis. • Simple flow sheet and flash regeneration system. • Low solubility for CH4, other hydrocarbons, H2 etc.

• Not suitable in presence of H2S. • Degradation of solvent in presence of water at high temperature.

Selexol Process:

• Initially developed by Allied Chemical Corporation and now licensed by UOP• Initially intended for bulk CO2 removal; other application is desulfurization• Selective removal of H2S can be achieved • Chemically stable, non-toxic and bio-degradable

• High solubility of methane and other hydrocarbons• Complex flow scheme for removal of H2S


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Purisol Process:

• Developed and commercialized by Lurgi GmbH.• Used for feeds containing high percentage of H2S• Useful for removing COS, CS2• Also used for recovery of butadiene from C4 and acetylene from pyrolysis gas

• Low vapour pressure; loss of solvent is high.

Rectisol Process:

• First organic physical solvent process for CO2 capture from synthesis gas• Process operates at -25 oC; high solubility of CO2 and other impurities• Better heat and mass transfer characteristics• Ability to removal HCN, aromatics, organic sulfur compounds etc.

• Complex flow scheme and complex design• Higher operating cost due to refrigeration requirement.


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Chemical solvents for CO2 absorption

– Carbonate buffer K2CO3-KHCO3

– Primary akanolamines

– Secondary akanolamines

– Tertiary akanolamines

Monoethanolamine HO-CH2-CH2-NH2

Diethanolamine HO-CH2-CH2-NH-CH2-CH2-OHDIPA (CH3-CH2OH-CH2)2-NH

Triethanolamine (HO-CH2-CH2)3-NMDEA (HO-CH2-CH2)2-N-CH3


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Alkanolamines for for CO2 absorption

OHOH

CH3

N

OH

NH2

Monoethanolamine

OHNH

OHDiethanolamineTriethanolamine

OHOH

NHCH3CH3

CH3

OHN

OH

Diisopropanolamine

OHO

NH2

MethydiethanolamineDiglycolamine


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Criteria for chemical absorbent selection• Temperature dependency of equilibrium• Absorption rate:

– Reaction rate– Diffusion rate

• Capacity:– High loading in absorber– Low loading in regenerator

• Heat of reaction should be low• Chemical stability• Foaming (interfacial tension)• Volatility• Price• Toxicologi• Degradability in nature


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Basic chemistry for chemical solventsPrincipal reactions for CO2 absorption in carbonate buffer

Ionization of water

2H2O = H3O+ + OH-

Hydrolysis and ionization of dissolved CO2

CO2 + 2H2O = HCO3- + H3O+

Dissociation of carbonate ion

CO32- + H2O = HCO3

- + OH-

Reaction with OH-

CO2 + OH- = HCO3-


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Commercial processes using carbonate buffer

Benfield Process:Process based on carbonate buffer solution and developed by U.S. Bureau of mine in early1960s. Operates at high temperature and high partial pressures of CO2. Typical energy consumption is 2400-2600 MJ/ton CO2. High selectivity, absorption rates compared to physical solvents, reduced plant size.

Problems: Precipitation of bicarbonates at higher conversions. Corrosion problems with carbon steel.Low absorption rate at low partial pressure of CO2

Catacarb process:Variant of Benfield process, introduced by Eickmeyer in 1962. Processes uses catalyst for rate enhancement such as primary amines for non-oxidative environment and inorganic catalyst for oxidative environment.


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Medium to highpressure in absorber

Pressure release before regeneration

Benfield Process (2400-2600 MJ/ton CO2)


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Medium to highpressure in absorber

Pressure release before regeneration

Split stream operation

HOT

Benfield Process with heat integration (1000-1500 MJ/ton CO2)


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Alkanolamines

Principal reactions for CO2 absorption in primary and secondary alkanolamines

Ionization of water

2H2O = H3O+ + OH-

Hydrolysis and ionization of dissolved CO2

CO2 + 2H2O = HCO3- + H3O+

Protonation of alkanolamine

RNH2 + H+ = RNH3+

Carbamate formation

RNH2 + CO2 = RNH2+COO- + B = RNHCOO- + BH+

2RNH2 + CO2 = RNHCOO- + RNH3+

Principal reactions for tertiary alkanolamines

CO2 + H2O-R3N =HCO3- + R3NH+


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Basic chemistry for chemical solventsOverall rate of reaction for CO2 absorption in primary and secondary alkanolamines based on the so called zwitterion mechanism

2

2 2 1

1

[ ][ ][ ]

[ ]

1[ ]

bi

biCO

bi

k BHk CO Am k RNHCOO

k BR k

k B

+−−

−

−

⎡ ⎤− ⎣ ⎦=

+

∑∑

∑


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Commercial processes based on alkanolamines

MEA Process: This system is preferred for streams with low concentrations ofCO2 or H2S and for maximum removal.

Advantages:High loading capacity on the basis of solvent weight.Faster rate of reaction.Possibility of simultaneous dehydration

disadvantages:Cannot be used when in presence of other contaminants e.g. COS, CS2Corrosive and degradation of MEALimited loading on mole basisHigh heat of reaction and high vapour pressure

Commercial process:1. Fluor Econamine and Econamine Plus2. GAS/SPEC FT by Dow chemicals3. Amine guard system by UOP


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DEA Process: This system is preferred for streams containing other contaminants like COS and CS2.

Advantages:High temperature and low pressure application possibleFaster rate of reaction.Less corrosive nature of DEAHigher loading on mole basis (0.7-1.0)

disadvantages:Irreversible side reactions with CO2

Commercial process:1. SNPA-DEA

DGA Process: Fluor Econamine process can be based on DGA. High concentration of amine (40-60 wt%) results in high rates of absorption and low circulation of solvent.


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MDEA Process: Initially was used for selective absorption of H2S from coal gasification products and for sulfur production. CO2 absorption is also possible with the help of activator.

Advantages:High temperature and low pressure application possibleLow energy requirementsHigh loading capacity, excellent stability etc.Less corrosive nature

disadvantages:Low rate of reaction with CO2

Commercial process:Activated MDEA by BASF (0.1-0.4 MMEA and 0.8 M piperazine)Praxair MDEA with MEA


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New solvent systems.

Mitsubishi: KS1, KS2 og KS3Reductions in energy consumption:

30% compared to MEA


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Other absorbents

Amit Chakma, University of Regina (Canada).

PSR1 og PSR2


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Typical absorption towerlay-out

Hydraulic considerations


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Loading (Billet 1995)

At a certain gas velocity the liquid at the g/l interface will start moving upwards:

We have then reached loading as shown by the bend in thediagram.

0.5 0.5 1/3 1/ 6, 1/ 6

12 12( ) ( ) ( )l l lv s l l

s l l v

gu a u ua g g

η η ρεξ ρ ρ ρ

⎡ ⎤= − ⋅⎢ ⎥

⎣ ⎦

Resistance factor:

2

2 0.4/( ( )sn

l ls s

v v

Lg CV

ρ ηξ

ρ η⎡ ⎤

= ⋅ ⋅⎢ ⎥⎣ ⎦


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Flooding occurs when the resistance to downward liquid flow is so high that no liquid runs through the column.

The pressure drop then increasesdramatically and no operation is possible

Flooding


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Liquid entrainment

When the gas velocity becomes large, then liquid, as droplets, is carried with thegas flow upwards. This leads to back-mixing in the tower, and eventually tocarryover of liquid into the outlet gas stream.


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Pressure dropThe pressure drop in a tower packing increases with the liquid load


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WettingGood wetting is important for the efficiency of the packing. From experiments ahydraulic area and a mass transfer area can be found, and related to the nominal dry area of packing.

2 20.5 0.2 0.75 0.45 0.5 0.2 0.75 0.453 ( ) ( ) ( ) 3 (Re ) ( ) ( )ph l l l l l

l l ll l

a u u u a We Fra a a g

ρ ρε ε

η σ− − − −⋅ ⋅ ⋅

= =⋅ ⋅


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Modeling of an absorber

CA

Ni

x=0 Position x

Phase 1Phase 2


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Modeling of absorber: Macroscopic look

• For simple geometries velocity distribution is known; flux calculation is easy.

– Laminar flow– Plug flow– Stagnant liquids

• For non-ideal flow simplified models using empirical parameters can be applied– Dispersion model– Tanks-in-series model

• Heat effects need to be taken into account for highly exothermic/endothermic reactions/processes

• Underlying models for equilibria, kinetics and transport properties


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Plug flow model for physical absorption• Plug flow (flat velocity profile) with no chemical reaction• Steady state operation

A differential balance over an element dz :

dz

(GCgi)z

(GCgi)z+dz(LCli)z+dz

(LCli)z

(Ni a dz)

( ) ( ) .gi li id GC d LC N a dz= =

The transfer flux can be expressed by the gas and liquid side mass transfer coefficients respectively:

, , , ,( ) ( )bulk if if bulki g g i g i l l i l iN k C C k C C= − = −

At the interface we assume equilibrium characterized by Henry’s law

, , ,if if ifg i g i i l iC RT P H C= =


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Solving the two flux equations for the driving forces and adding, introducing Henry’s law gives:

( )*, , , ,

11

bulk bulk bulki g i l i l l i l i

g l

RTN C C K C CRT HHk k

⎛ ⎞= − = −⎜ ⎟⎝ ⎠+

The differential material balance can then be written as:

( ) ( )*, , ,bulk bulkl i i l l i l id LC N adz K a C C dz= = −

We reorder the equation and integrate, assuming that the liquid flow rate is constant:

( )( )

,

*0 , ,

bulkZ coutl i

bulkcin l l i l i

Ld CZ dz

K a C C= =

−∫ ∫


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, ,, , , ,

,

' '(1 ) (1 )

bulkg i topbulk bulk bulk

l i top g i l ii i top

CGLC C LC Gy y

+ = +− −

*, , ,

bulk bulkg i g i tot i i l iC RT p p y H C= = =

To integrate this equation we need to know the total mass transfer coefficientand we need a relationship to get *

,l iC

G’ = inert flow. Using ideal gas low or EOS

(LCli)top (GCgi)topA material balance over the top of the column:

(GCgi)z(LCli)z

*, ,,

, , ,*, ,(1 / ) ' (1 ) '

bulkg i topi l i bulk bulk

l i l i topi l i tot i top

CH C L LC CRT H C p G y G

= + −− −

Giving:

This equation gives a relationship between Cl,i* and Cl,i

bulk which can be used in the integral we developed before.The integral can then be solved and the height of packing, Z, found


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Introducing chemical reaction

We can also for chemical reaction use the same procedure and end up with the same integral:

,,

* ,, ,0

( )( )

bulk freeZ coutl i

bulk freel l i l icin

L d cZ dz

K a c c⋅

= =⋅ ⋅ −∫ ∫

However, now the interpretation of some of the variables will be different.Say, the reaction taking place is:

A bB cC+ ↔The component A is the gas being transferred, and if the chemical reaction is more or less irreversible, as may often be the case, then most of the A will be in chemically bound form. However, only the free A will give a contribution to the partial pressure through Henry’s law.

*, , /( )bulk

g A A l Ac H c R T= ⋅ ⋅ *,A A l AP H c= ⋅or


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The total amount of A in the liquid phase will be:total free boundA A Ac c c= +

The fictitious liquid phase concentration of free A, cA*, can be found

through a material balance as before:

, , ,, ,, , ,

, ,

)(1 ( ) / ) ' (1 ) '

bulk bulkg A g A topbulk total bulk total

l A l A topbulkg A tot A top

c cL Lc cc R T p G y G

= ⋅ + − ⋅− ⋅ ⋅ −

The A-concentration in this equation is however the total one.In other words, this equation now gives a relationship between the bulk gas phase concentration of A and the bulk total liquid phase concentration of A.

In the Henry’s law, however, the free liquid phase concentration of A enters.Conclusion:

We need a model to connect cAtotal to cA

free : an equilibrium model


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Mass Transfer models Mass transfer models

Film model

Penetration model

Surface renewal model

Film-penetration model

Interfacial Turbulence model……………………….

In general we cannot calculate/measure local velocities required in micro-mass balance

(except for stagnant media, laminar flow,…)

Are we finished now ??

NO: we assume local flow behaviour near interface

One parameter

models

All mass transfer models use mass transfer coefficient km that accounts for the unknown micro-convective flow

Dependence of mass transfer coefficient on diffusion coefficient depends on model:

)C(CJk

interfaceA,bulkA,

Am −

=


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Assumptions:Concentration profile is located in small film layer on both sides of G-L interface (δ1 and δ2)Ideally mixed bulk phasesFilm layer is stagnant and stationaryG-L interface at equilibrium

Micro mass balance becomes:

Boundaryconditions:

n2nA,

2

nA, δx0xC

D0 <<=∂

∂

nbulk,nA,n

ninterface,nA,CC:δx

CC:0x==

==

bulkA,A

interfaceA,A

bulkA,A

CC:x

CC:0xttx0C (x)C:0t

=∞=

===

∞<<==

Film model

x

CA,2bulk

CA,i,2

δ2δ1

CA,i,1

CA,1bulk

Phase 2Phase 1

Penetration model

Phase 1

Phase 2

JA

Increasing contact time τ

At t=τ s package returns to bulk

At t=0s a package comes from bulk

Assumptions:Assumes that stagnant packages with infinite thickness go from bulk to the interface and uniformly return to bulk after t seconds

Micro mass balance becomes:

Boundaryconditions:

2A

2

AA

xCD

tC

∂∂

∂∂

=


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Film model Penetration model

)C(CJk

interfaceA,bulkA,

Am −

=

⎥⎦

⎤⎢⎣

⎡ −−=⎥

⎦

⎤⎢⎣

⎡∂

∂−=

= n

interfaceA,bulkA,nA,

0x

nA,nA,A

CCD

xC

DJδ

)C(Cπt

Dx

CDJ bulkA,iA,A

0xA

AA −=⎟⎠⎞

⎜⎝⎛−=

=∂∂

Solution can be derived analytically for micro-balance equation

Momentary flux

Contact time averaged flux

)C(Ck)C(C π

D2dtJτ1J bulkA,iA,mbulkA,iA,

Aτ

0AA −≡−== ∫ τ

τ πD2k A

m =

nm,nnA, k

δD

=⎟⎟⎠

⎞⎜⎜⎝

⎛

For the film model the mass transfer coefficient is proportional to the diffusion coefficient

For the penetration model the mass transfer coefficient is proportional to the square root of the diffusion coefficient


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Effect of chemical reaction on mass-transfer

Reaction can occur in mass transfer layer

Reaction cannot occur inside mass transfer layer

Mass transfer and reaction are in seriesConcentration profiles in mass transfer

layer is not affected

Mass transfer and reaction are in parallelConcentration profiles in mass transfer

layer is affected

Simultaneous reaction can enhance flux

CA,2,bulk

δ1 δ2

CA,1,bulk

CA,1,i

CA,2,i

Phase 1: No reaction,

using film modelPhase 2:

Reaction, using film model

RA i

AA x

CDJ ⎟⎠⎞

⎜⎝⎛−=

∂∂

Mass balance over film phase 1

Mass balance over film phase with reaction in film

( )iA,1,bulkA,1,1

A,1A CC

δD

J −=

22LA,

2

LA, δ x0RxC

D0 ≤≤−=∂

∂

AδA,A

Lbulk,LA,2

iA,1,Linterface,LA,

/DJdx

dCorCC:δx

mCCC:0x

−===

===


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Definition of Enhancement factor EA and Ha

)C(Ckx

CDJ bulk2A,i,2A,Li

AA −=⎟

⎠⎞

⎜⎝⎛−=

∂∂

iA

reactionA, xCDJ ⎟

⎠⎞

⎜⎝⎛−=

∂∂

No reaction:

Reaction:

Enhancement factor

( )LA,Li,A,LchemicalA,

physicalA,chemicalA,

A CCkJ

JJ

E−

==

Compare fluxes at identical driving force !!

filmthroughtransportmaximumfilm in conversion maximumHa =2

Effect of reaction on mass transfer can be analyzed by taking ratio of two extreme situations

( )0

2

−=

i,AA

b,bi,A1,1

CDCCK

Ha

δ

δ

δA

LDk =Remember the definition of

L

b,b1,1A

kCKD

Ha =

For (1,1) reaction

For (m,n) reaction

L

nb,b

mi,Anm,A

k

CCKD1m

2

Ha1−

+=physicalA,AchemicalA, JEJ =


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Very slow reaction regimes:In the very slow regime all reaction takes place in the bulk of the liquid and the diffusional processes are so fast compared to the reaction rate that no gradients occur.

Effect of chemical reaction on mass-transfer: reaction regimes

δG

δL

CA,1,b

CA,G,i

CA,2,b

CB,b

Saturated bulkLB,Li,A,1,1A CCkaJ =

δG

δL

CA,1,bulk

CA,G,i

CB,bulk

CA

EA = 1, CA,L=0

Slow reaction regimes:Rate of reaction is faster than diffusionaltransfer of ‘A’ to phase ‘B’. Reaction occurs uniformly throughout ‘B’ but the rate is controlled by the transfer of ‘A’. Practically no reaction occurs in film.

1<<= Ha;aCkaJ Li,A,LA


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Fast reaction regimes:The reaction occurs while ‘A’ is diffusing in the film. Diffusion and chemical reaction are parallel steps. The reaction is so fast that concentration gradients occur in the film, thus leading to rate enhancement.


δG δL

CA,1,b

CA,G,i

CB,b

Pseudo first order: EA = HaA2>= Ha;aHaCkaJ Li,A,LA

Infinitely fast reaction regimes:The reaction is so fast that the solute and the reactant cannot coexists together. A reaction plane is formed and mass transfer is governed by the diffusion of ‘A’ and ‘B’to the reaction plane.

δGδL

CA,1,b

CA,G,i

CB,b

CA

Infinite enhancement: EA =EA,∝

δr

2>== ∞ Ha;CδDCEkJ Li,A,

r

ALi,A,A,LA


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δGδL

CA,1,b

CA,G,i

CB,b

CA

Infinite enhancement: EA =EA,∝

δr

Li,A,rA

A CδDJ = LB,

rLB

B Cδ-δ

DJ =

JA=JB

Li,A,A,LLi,A,A

LB,BLi,A,

LA

A CEkCDCD

1CδDJ ∞=⎥

⎦

⎤⎢⎣

⎡+=

Li,A,ALB,B

A, CDCD

1E +=∞Reaction so fast that A and B cannot co-exist. The flux cannot get any higher than this !!


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Effect of chemical reaction on mass-transfer: reaction regimes-summery

Regime AHa CA,bulk EA JA

Very slow < 0.3 ~ CAi 1 r

LbulkB,iA,1,1

LA,iA,LA

aVVCCk

)C(CkJ

=

−=

Slow <0.3 0 1 Li,A,LA CkJ =

Fast 2<HaA <<EA,∞ 0 HaA Li,A,AbulkB,1,1

Li,A,ALA

CDCk

CHakJ ==

Instantaneous HaA >>EA,∞ 0 EA,inf Li,A,

Li,A,A,bulkB,B

L

Li,A,A,LA

)CCD

CD(1k

CEkJ

+

== ∞

For transition regions enhancement factor needs to be calculated numerically !!!


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Effect of chemical reaction on mass-transfer: E vsHa plot

HaA < 0.3EA = 1

2 < HaA << E A,∞EA = HaA

HaA > E A, ∞EA = E A, ∞

E A= H

a A

⎟⎟⎠

⎞⎜⎜⎝

⎛+

−+

−+

−−

=

∞

∞

∞∞

11)(E

HaE1)4(E

Ha1)2(E

Ha

E

A,

2AA,

2A,

4A

A,

2A

A

General approximation of EA for (1,1) reaction

We need to regenerate the solvent so what about reversible reactions ?

vAA + vBB ↔ vCC + vDD

Reversible reaction mainly affects EA,∞

Keq EA,∞


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How to calculate overall flux in presence of chemical reaction?

The flux for the liquid phase can be calculated using

The flux for the inert gas phase is given by:

At interface local equilibrium holds: CAi,L=mCAi,G

EA and CA,L are a function of HaA and EA,∞

Remember: If EA and CA,L cannot be determined using asymptotic analytical solutions numerical tools are required

( )LA,Li,A,ALchemicalA, CCEkJ −=

( )GA,Gi,A,GGA, CCkJ −=

Continuity:J A,chemical = JA,G

J

k m k E

CC

mA

G L A

A GA L=

+−

⎛⎝⎜

⎞⎠⎟

11 1 ,

,


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Equilibrium model

H2OMolecularelectrolytes

Ionicspecies

H2OMolecular electrolytes

Vapor

Liquid

Interface


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Phase Equilibria

Thermodymically equilibrium is possible when dG = 0

G = f(T, P, chemical potential)

Hence, at equilibrium

TV = TL; PV = PL; µVi = µL

i

Using reference state chemical potential

µi0 + RTlnfi

V = µi0 + RTlnfi

L

For same reference state

fiV(T, P, yi) = fi

L(T, P, xi)


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Phase Equilibria

The vapor phase fugacity for real solution

fiV(T, P, yi) = Pyiϕi

where ϕi is fugacity coefficient and can be calculated from residual function and EOS

The liquid phase fugacity for real solution

fiL(T, P, xi) = fi

ref(T, P, xiref)xiγi

where γi is activity coefficient and can be calculated from excess free energy GE.


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Importance of reference stateThe calculation of thermodynamic properties of an ideal solution is relatively easy.

For ideal solution fidi = fref

i xi

For real solution fi = frefi xi γi

Pure component reference state-Raoult’s Lawfi (T, P, xi) = f•

i(T,P) xi γi

Usually applied to the dominating component in the solution.In an ideal solution application of pure component reference state results in Raoult’s law.

Pi = PiS xi


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Importance of reference state

Infinite dilution reference state-Henry’s Law

fidi(T, P, xi) = Hi(T,P) xi

fi(T, P, xi) = Hi(T,P) xi

Usually applied to calculate gas solubilities. In an ideal solution application of infinite dilute reference state results in Henry’s law.

Pi = Hixi

iγ∧

lim0 1ix γ

∧

→ =0 xi 1

fi f •i

Hi

Henry’s law

Raoult’s law


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Chemical Equilibria

Chemical equilibrium governs the extent of dissociation and reaction and so the distribution of species.

The equilibrium condition stoichiometric formulation

1

0n

i ii

v µ=

=∑

Traditionally, the chemical equilibrium is defined by equilibrium constant ‘K’

1 1

i i i

n nv v vi i i

i iK a xγ

= =

= =∏ ∏


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System CO2-MEA-H2O

• Chemical equilibria

2 32H O H O OH+ −+

2 2 3 32H O CO H O HCO+ −+ +

2 3H O MEAH H O MEA+ ++ +

22 3 3 3H O HCO H O CO− + −+ +

2 3H O MEACOO HCO MEA− −+ +

3 3

2

2 2

2

( ) ( )

( )H O H O O H O H

H OH O H O

x xK

x

γ γ

γ+ + − −⋅

=

3 3 3 3

2

2 2 2 2

( ) ( )

( ) ( )H O H O H C O H C O

C OH O H O C O C O

x xK

x x

γ γ

γ γ+ + − −⋅

=⋅

3 3

2 2

( ) ( )

( ) ( )M E A M E AH O H O

M E AH O H O M E AH M EA H

x xK

x x

γ γ

γ γ+ +

+ +

⋅=

⋅

2 23 3 3 3

32 2 3 3

( ) ( )

( ) ( )H O H O C O C O

H C OH O H O H C O H C O

x xK

x x

γ γ

γ γ

+ + − −

−

− −

⋅=

⋅

3 3

2 2

( ) ( )

( ) ( )MEA MEA HCO HCO

MEACOOH O H O MEACOO MEACOO

x xK

x x

γ γ

γ γ− −

−

− −

⋅=

⋅


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System CO2-MEA-H2O

• Mole balances

22 2 3 3

totH O H O HCO OH CO

n n n n n− − −= + + +

22 2 3 3

totCO CO HCO CO MEACOO

n n n n n− − −= + + +

totMEA MEA MEAH MEACOO

n n n n+ −= + +

2 23 3 3 3 3 3

0OH OH HCO HCO CO CO H O H O MEAH MEAH MEACOO MEACOO

n n n n n nν ν ν ν ν ν− − − − − − + + + + − −= + + + + +

• Electroneutrality


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System CO2-MEA-H2O

Phase equilibria for CO2

( )2 2

2 2 2 2 2exp

sCO H O

co co CO CO CO

V P Py P x H

RTϕ γ

∞∧

∞⎛ ⎞−

= ⎜ ⎟⎜ ⎟⎝ ⎠

Phase equilibria for solvent

( )exp

s sS Ss

S S S S S S

V P Py P x P

RTϕ γ ϕ

⎛ ⎞−= ⎜ ⎟

⎝ ⎠


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Liquid phase - SPECIATION

Vapor phase - BUBBLE POINT CALC.

Computational Scheme

T, αCO2, camin

x - liquid phasecomposition

γ - activity coefficients

MSA

T, x, γ

y - vapour phasecomposition

ϕ - fugacity coefficients

PR-EOS

p, y

x, γ

Simultaneous solution of: chemical equilibrium, mole balances, and electroneutrality

Phase equilibriafi

L = fiV

pi = fiV/ ϕI

p=∑ pi

x = x0 , γ =1

p = 0 , ϕ =1


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Thermodynamic models

Models for CO2 in Alkanolamines

• Austgen, Rochelle (1989) [Chen’s NRTL ]

• Kent, Eisenberg (1976) [apparent Keq]

• Deshmukh, Mather (1981) [Guggenheim]

• Li, Mather (1994) [Pitzer]

• Lee (1991-1995 ) [ElcGC]


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Fit to Binary VLE-Data

0 0.2 0.4 0.6 0.8 10.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

mole fraction of H2O

gam

a

gamaH2O UNIFACgamaMEA UNIFACgamaH2O (wilson)gamaMEA (wilson)gamaH2O (orig. UNIF)gamaMEA (orig. UNIF)

0 0.2 0.4 0.6 0.8 10

50

100

150

mole fraction of H2O

Ptot

[tor

r]

UNIFACRaoults lawPexpl

Total pressure Activity coefficients


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Vapour-Liquid Equilibrium

10-2 10-1 10010-6

10-4

10-2

100

102

104

loading (mol CO2/mol MEA)

PC

O2

(kP

a)

5.0M MEA 313K modellgam=1Jou and Mather(1995)Shen and Li(1992)Lee, Otto and Mather (1974)

10-2 10-1 10010-6

10-4

10-2

100

102

104

loading (mol CO2/mol MEA)

PC

O2

(kP

a)

2.5M MEA 313K

modelmodel (gam=1)Lee Otto and Mather(1974)Lee Otto and Mather(1976)Lawson and Garst (1976)


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100

101

102

103

104

10−4

10−3

10−2

10−1

100

101

102

103

104

CO

2 pr

essu

re, k

Pa

CO2 molar conc.,mol/m3

40C,mech.mod.70C,mech.mod.100C,mech.mod.120C,mech.mod.40C,Jou et al 198240C,Austgen et Rochelle 199240C,Xu et al 199270C,Jou et al 198270C, Nilsen 2002100C,Jou et al 1982120C,Jou et al 1982

Sample equilibrium data for MDEA


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Equilibrium and kinetics• “Speciation”: Concentrations of all components are calculated from the

equilibrium model• Chemical reaction:

)CC(Ckr eq

COCOMEACO 2222 −−=


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Membrane absorbers:

• TNO:– Polypropylen membrane (not amine-resistent)– Chemical (CORAL) more resistant to oxygen than MEA– Energy consumption as for MEA– Mass transfer rates better than MEA– Inorganic chemical with no evaporation and low

degradation losses

• Kværner:– PTFE-membrane (Gore) (resistent against most systems,

incl. aminer)– Can use the Mitsubishi and Chakma systems– Advanced in module construction– Demonstrated in pilot scale


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Rich solvent

Raw gas inTreated gas out

Lean solvent

The Kvaerner/Gore membrane processPTFE has excellent properties as membrane material

Hydrophobicity

Long term stability

Hollow fiber membrane with liquid flow inside fiber

Applications developed for both natural gas and exhaust gas CO2 removal (figure)


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Gas absorption membrane:

CO2

Flue Gas

CO2

MicroporousMembrane

Absorption Liquid

Membrane not selective, only separates the phasesDiffusion through pores followed by reaction in liquid; CO2-absorption with alkanolamine solutions


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Why Membrane Gas Absorption ?• Advantages

Ease of operationHigh mass transfer areaEasy scale upHydrodynamic flexibility Modular, compact design

• DisadvantagesMembrane resistanceShell side mal-distribution Membrane fouling, wettingFinite membrane life


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Basics of simulation model

Concentration profile

z

ngCO2

nginert

rGas

Liquid

velocity profile

NCO2NH2O

Membrane

AssumptionsPlug flow of gas Parabolic liquid velocity profile Counter- or co-current flowGas filled membrane with purely diffusional transportConstant compressiblity and fugacity coefficients

SubmodelsEquilibriaKineticsTransport properties


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Diffusion-reaction model for liquid phase

Boundary conditions0

i iz = 0 = CC

iCr = 0 = 0r

δδ

ii i

Cr = R = ND rδδ

Transfer flux model

ii

Cr = R - = 0 D rδδ

peff

M

DD

ετ

=

For diffusing species:

For non-diffusing species:

Effective diffusivity:

( )1 ( )ln( )1 i iy y i

g eff

N p Hc ii R R Rk D

= −+

r rD

rC ))

rC(r

r

r1(D =

zC])

Rr( - 1 [v2 i

iiii

i2avz, ++⋅⋅⋅⋅

δδ

δδ

δδ

δδ

δδ


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Balance equations for gas phase

Linear gas velocity

εδδ

δδ

δδ

δδ

gii

g

g2

ggggg

tot aN- =

zT

TRZ

vP - zv

TRZP +

zP

TRZv =

zn ⋅Σ

⋅⋅

⋅⋅⋅⋅⋅

Partial pressure of gas components

εδδ

δδ

δδ

δδ

gi

g

g2

g

i

gg

ii

gg

i aN- =

zT

TRZ

vP - zv

TRZP +

zP

TRZv =

zn ⋅

⋅⋅

⋅⋅⋅⋅⋅


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3D-profiles (MEA-case)

Free CO2 Bound CO2


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3D-profiles (MEA case)

Viscosity Density


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Sample results, experiments compared to model

Laboratory unit30 wt% MEA, 5 wt% PZ

Pilot plant15 wt% MEA

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

0 0.1 0.2 0.3 0.4 0.5 0.6

CO2 loading (mol CO2/mol MEA)

RC

O2 (

mol

/s)

ExperimentsSimulations

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

3.00E-03

3.50E-03

4.00E-03

4.50E-03

5.00E-03

0 0.2 0.4 0.6 0.8 1cCO2 (mol/l)

RC

O2

(mol

/s)

Exp. dataSimulations


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Laboratory pilot facility

Combined column and membrane absorption set-up

15 cm ID absorber, 4.5m10 cm ID desorber

Can run with membrane absorber and/or desorber.

Fully instrumented and automatedto run on a 24 hour basis


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References

Billet R., “Packed Towers”, VCH, Weinheim 1995

Strigle R.F., “Packed Tower Design and Applications”, Gulf Publishing Co.,2nd. Ed., 1994

Kohl A., Nielsen R., “ Gas Purification”, Gulf Publishing Co., 5nd. Ed., 1997

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