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