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 [email protected]
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
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