Simulation of CO2 capture

with Solid sorbents using Aspen Plus constructs

J.W. Dijkstra

April 2016

ECN-L--16-016

www.ecn.nl

Simulation of CO2 capture with

Solid sorbents using Aspen Plus

constructs Jan Wilco Dijkstra

Petten KOPSE Meeting

4th april 2016

Post-combustion CO2 capture

for power stations

Conventional technology for CO2

removal from flue gas

• Chemical solvents

• High regeneration heat – Reaction heat

– Sensible heat

– Water evaporation

• Significant solvent use/waste

• Nitrosamine emissions?

Solid sorbents as an alternative

Potential benefits • Low Cp ~3.8 vs ~1.1 kJ/(mol.K) • Less water evaporation Only adsorbed water • Less degradation • Less emissions

Disadvantage • (Probably) no regenerative heat exchanger • Novel

Supported polyamines

PEI = Polyethyleneimine

Polymer impregnation procedure on commercial porous silica material

Objective and approach

Sorbent preparation & characterization

Systems model development

Development goals Optimum conditions Potential

isotherms

Objective: assessment of working conditions, energy savings potential and development goals

Isotherms from break-through

experiments

CO2

CO2

k.p1

k.pΘ

H2Ok.pΘ

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 0.02 0.04 0.06 0.08 0.1 0.12

Wat

er

bre

akth

rou

gh C

apac

ity

[mm

ol/

g]

H2O pressure [bar]

60°C

70°C

80°C

90°C

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.05 0.1 0.15 0.2 0.25 0.3

CO

2b

reak

thro

ugh

Cap

acit

y (m

mo

l/g)

CO2 pressure (bar)

60°C

80°C

90°C

100°C

110°C

120°C

135°C

CO2: Langmuir H2O: Henry

Non-optimized sorbent: PEI on silica

6

6.5

7

7.5

8

8.5

9

9.5

10

0.0027 0.00275 0.0028 0.00285 0.0029 0.00295 0.003 0.00305

-ln

(KH

2O)

1/T [K-1]

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.00255 0.0026 0.00265 0.0027 0.00275 0.0028 0.00285

-ln

(K

CO

2)

1/T (K-1)

Temperature dependency

RT

Δhexpkk abs

0van’t Hoff’s law:

CO2

dH=-91 kJ/mol Water dH=-60 kJ/mol

Reactor concept

Multistage fluid bed adsorber Bubbling fluid be regenerator

Counter-current Cross-flow

BFB

Steam Cooling

CO2/H2O

Flue gas

Clean gas

Modeling platform: Aspen Plus

+ Good in mass/heat balances

+ Useful thermodynamic properties

+ Staged development

- No isotherms Add iteration blocks

- No suitable models for reactors Use ‘constructs’ of HX, splitters, mixers etc.

What are constructs?

Constructs are clever arrangements of simple blocks that allow you to model complex systems (Schad, 1998)

Example 1: Spray condenser

• No equilibrium achieved

- => flash vessel not possible

• Use rigorous distillation with low

stage efficiency

Ryan C. Schad, Make the most of process simulations, Chem. Eng. Prog., jan 1988. 21-27.

Example 2: Membrane with sweep

Hydrogen

hydrogen Feed

Sweep

Retentate

Permeate

DESIGN SPEC e.g. TARGET : (X_FEED*P_FEED)-(X_PERM*PPERM) VALUE : xx bar VARY : P_PERM Optionally * Add also equations for membrane surface area * Add HX’s for heat exchange

SEP1

MIX1

FEED

RET

SWEEP PERM

MEMH2

Driving force= partial pressure difference pH2

Feed>pH2perm

Modeling approach

Adsorber

Flue gas

CO2+ H2O

Regenerator

Clean gas

Rich solid sorbent

Lean solid sorbent

Assume equilibrium between gas and solid at outlet conditions

Design strategy:

Adsorber

Flue gas

CO2+ H2O

Regenerator

Clean gas

Rich solid sorbent

Lean solid sorbent

2. Adapt solids flow rate to saturate rich sorbent

1. Target % CO2 in clean gas Dictates leanness of sorbent Dictates regenerator temperature

3. Optimize absorption temperature with sensitivity study

Solids looping model

Adsorber section

Regenerator section

Flue gas

Cooling water

DCC SensibleSorbent/gas

DHCO2 DHWAT

SensibleSorbent/gas

DHCO2 DHWAT

Sensible water

Sensible water

Clean Flue gas

Total adsorption heat

Total regeneration

heat

Wet CO2 to compression

WaterLoaded sorbent

Lean sorbent

CO2

Important blocks

• INISPEC – CALCULATOR block with isotherm PARAMETERS used throughout the model

– Executed FIRST

• LEANCAP – DESIGN SPEC with FORTRAN

– Calculate sorbent loading based on Temperature and PCO2 of regenerator outlet (SL1)

– Calculate sorbent loading based on Temperate and PCO2 of absorber outlet (SL2)

- Vary split fraction of SPLITTER in regenarator SEPARATOR unit SL1/SL2=1

• RICHCAP – Similar one for the absorber outlet

• CCR – VARY temperature of regenerator until x_co2_stackgas=10%

When things get messy

• Adding water adsorption - Design on CO2, water follows

- ‘Pinch point’ in absorber can be on rich and lean side’ ==> IF/THEN needed

Use Excel (not recommended)

Install FORTAN compiler (not available)

Do something obscure with ROUND command

• Water, required as a separate stream, but condenses vapor only

• Convergence problems – 10 DESIGN SPEC, 9 CALCULATOR blocks makes that this requires some attention

– Assign CONVERGENCE blocks to loops to easily spot problems

– 2 DESIGN SPECS modified to NEWTON

– NESTING order specified for 4 CONVERGENCE blocks

40

45

50

55

60

65

70

75

80

0

1

2

3

4

5

6

7

8

60 65 70 75 80 85 90 95 100

De

ltaT

[°C

]

He

at [

GJ/

ton

CO

2]

Absorber temperature [°C]

Total regeneration heat

CO2 sorption heat

Sensible heat

Water sorption heat

DeltaT

Regeneration heat

Optimum

1.00

1.73

0.95

1.34 0.32

0.30

1.76

2.07

2.07

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

MEA reference Solid sorbent Solid sorbent 200% cap

Regeneration heat[GJ/ton CO2]

CO2 reaction heat

Water evaporation/desorption

Sensible heat reflux water

Sensible heat sorbent/solvent

Total 4.26 Total 4.13

Total 3.31

Comparison with MEA

CO2 reaction

Water evaporation /desorption

Sensible heat Q=cp T

Solid sorbent MEA

Improved sorbent

4.96

5.28

4.68

3.69

4.00

3.51

4.17 4.184.07

3.243.37

3.16

0.0

1.0

2.0

3.0

4.0

5.0

6.0

CO2 100%H2O 100%Base Case

H2O 200% H2O 50% CO2 200% CO2 200%H2O 200%

CO2 200%H2O 50%

Re

gen

era

tio

n h

ea

t [G

J/to

n]

CO2 and H2O capacity relative to base case

NGCC case

PC Case

Conclusions/evaluation Aspen Plus

• Constructs are very useful, use them in your simulations!

• Really complex constructs like the sorbent model Good if you are familiar with ASPEN PLUS, are not familiar with something else

• You benefit from – Thermodynamic data, or want to integrate with a flow sheet

– Staged development, you can start really easy

– Control over convergence

• If not you can consider: – Aspen Custom Modeler

– gPROMS

– Matlab (Simulink)

Conclusions solid sorbents

• CO2 solid sorbents interesting as a 2nd generation post-combustion sorbent – Capacity required at least twice that of non-optimized sorbent

– Advantages low Cp and limited water sorption

• To arrive at these conclusions we need a combined modeling and experimental approach – Insight in optimal working conditions

– Insight in advantages, development goals and potential

Thank you

ECN

Westerduinweg 3 P.O. Box 1

1755 LE Petten 1755 ZG Petten

The Netherlands The Netherlands

T +31 88 515 49 49 info@ecn.nl

F +31 88 515 44 80 www.ecn.nl

Contributers: Stephane Walspurger, Gerard Elzinga, Rick Reijers, Miranda Heijink-Smit Özlem Pirgon-Galin, Jurriaan Boon, Wim Haije and others

Graphical representation of

cyclic process

0

0.5

1

1.5

2

0 200 400 600 800 1000

Load

ing

CO

2,H

2O [

mm

ol/

g]

Partial pressure (CO2, H2O), [mbar]

H2O, des, 138°C

CO2, abs, 86°C

CO2, des,138°C

H2O, abs, 86°C

8

ECN

Westerduinweg 3 P.O. Box 1

1755 LE Petten 1755 LG Petten

The Netherlands The Netherlands

T +31 88 515 4949

F +31 88 515 8338

info@ ecn.nl

www.ecn.nl