physical and chemical equilibrium of co2-water-mineral system using aspen plus process simulator...
TRANSCRIPT
Physical and chemical equilibrium of CO2-Water-Mineral system using
Aspen Plus process simulator
Technical University of Delft
Ali Akbar EftekhariHans Bruining
Outline
Application of thermodynamics in CO2 sequestration
Process simulators: why?
Phase equilibrium of CO2-Water mixture: How accurate are the thermodynamic models?
Tuning the parameters of thermodynamic models using experimental data
Phase and chemical equilibria of CO2-Water-mineral system
Concluding remarks
CO2 sequestration in aquifers
CO2 diffusion in water enhanced by natural convection
C O 2
V a p o r-L iq u id E q u ilib ria
W a te r
Effect of solubility prediction on the simulation results
2 2 8 m o l/m ^3
2 4 7 m o l/m ^3
Run time: 60000 s
Henry's constant in the right hand side simulation is just 10 % lower than the left hand side case.
Other assumptions:-Ideal gas law-Ideal liquid
Process Simulators A comprehensive database:
Physical properties of pure components and binary mixtures
Thermodynamic models
Equation of state, activity coefficient models, etc
Very powerful algorithms in the calculation of fluid phase equilibrium
Mathematical model
simple mass and energy balance (not momentum)
A beautiful and user-friendly GUI
Fluid phase equilibria
Equilibrium constraint: Chemical potential of
species i in the vapor and liquid phase are equal.
Chemical potential can be evaluated using a thermodynamic model at known temperature and pressure.
Vapor phase (V)y
i
Liquid phase (L)x
i
T , P
Flash calculation
For Ideal gas law:
Vapor phase (V)y
i
Liquid phase (L)x
i
T , P
A ctv ity co e ffic ie n to f co m p o n e n t i in th e
liq u id p h a seF u g a c ity o f p u re
co m p o n e n t i
F u g a c ity co e ffic ie n to f co m p o n e n t i in th e
va p o r p h a se
For Henry's law:
Flash calculation
Soave-Redlich-Kwong
Peng-Robinson
Thermodynamic property models available in a simulator
Carlson, E. C., “Don't gamble with physical properties for simulations”, Chem. Eng.Prog., 1996
Process Simulators
A large set of up to date physical properties and phase equilibrium data combined with -often- old physical property models
A very well-written and debugged computer code with a user-friendly GUI
TIME
Process simulators
Calculation of physical properties for pure components and mixtures
Consistency tests Data regression
VLE results of Aspen Plus
0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.0000.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000 CO2 molality versus pressure at 50 degrees Celcius
Experimental CO2 molalityAspen (PR EOS)Aspen (PR after tuning)
Pressure (MPa)
CO
2 M
ola
lity
(m
ole
/kg
Wat
er)
Error
Error
Err
or
Time? How?
Accuracy of the models
Water-CO2 system Below critical temperature (T < 30 C)
LLE VLE
Over critical temperature (T > 30 C) VLE
Accuracy of the models
Peng-Robinson EOS Can't predict VLE and LLE data by its own
parameters After tuning:
Good fitting to the solubility of CO2 in liquid Poor fitting to the water vapor data points Not suitable for prediction of LLE
Accuracy of the models
NRTL (Non-Random Two-Liquid) with Henry's law
Acceptable prediction of the CO2 solubility in water with deviation at higher pressures
Cannot predict LLE by itself Acceptable results after tuning
VLE results of Aspen Plus
0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.0000.000
0.200
0.400
0.600
0.800
1.000
1.200
Experimental CO2 molality
NRTL-Henry (Aspen)
ELECNRTL-Henry (Aspen)
Pressure (MPa)
CO
2 M
olal
ity (
mol
e C
O2/
kg w
ater
)
Accuracy of the models
ELECNRTL-Henry-RK Accurate prediction of VLE data in both
phases Cannot predict LLE without tuning
Acceptable results after tuning Very accurate results after tuning, but over a
limited range of T and P
VLE data below the Tc
0 5 10 15 20 25 30 35 40
0.00000
0.00200
0.00400
0.00600
0.00800
0.01000
0.01200
0.01400
0.01600
0.01800
0.02000
VLE data at 25.13 C
x co2
y water
x co2 (calc)
y water (calc)
Pressure (bar)
mo
le f
ract
ion
4 6 8 10 12 14 16 18 20 22
0.00000
0.00200
0.00400
0.00600
0.00800
0.01000
0.01200
0.01400
0.01600
VLE data at 15.11 C
x co2
y water
x co2 (calc)
y water (calc)
Pressure (bar)
mol
e fr
actio
n
VLE data over the Tc
0 10 20 30 40 50 60 70 80
0.00000
0.00500
0.01000
0.01500
0.02000
0.02500
0.03000
VLE data at 45.08 C
x co2
y water
x co2 (calc)
y water (calc)
Pressure (bar)
Mo
le fr
act
ion
0 10 20 30 40 50 60 70 80 90
0.00000
0.00500
0.01000
0.01500
0.02000
0.02500
VLE data at 35.05 C
x co2
y water
x co2 (calc)
y water (calc)
Pressure (bar)
Mo
le fr
act
ion
Combination of NRTL and PR model after the tuning
2 4 6 8 10 12 14 16
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
VLE data at 50 degrees Celsius
x CO2
y Water
x CO2 calculated
y water calculated
Pressure (MPa)
mol
e fr
actio
n
Electrolyte solutions
CO2 solubility in the brine solution
0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
CO2 molality (mole/kg)Aspen Plus (ElecNRTL)
Pressure (MPa)
CO
2 M
ola
lity
(mo
le/k
g w
ate
r)
NaCl molality: 3.997 mole/kg waterTemperature: 40.01 C
Conclusion Phase equilibrium behavior of CO2-water/brine
system has a drastic effect on the simulation results of CO2 sequestration in aquifers
Aspen Plus (or other process simulators) can accelerate the procedure of property model selection, consistency test of experimental data, and tuning of the parameters
A combination of an electrolyte model, Henry's law, and a simple EOS can generate pretty accurate VLE results in the lack of experimental data
No property model can predict the VLE and LLE accurately on a very wide range of temperature and pressure
Thank you for you attention