Anthropogenetic emanations of CO2 to the ambiance have being identified as the major compound impacting the stableness of Earth clime [ IPCC, 2001 ] . Underground geologic storage of CO2 is, amongst other options, a feasible economic path to accomplishing the United Nations Framework Convention on Climate alteration ( UNFCC ) stabilization mark while still keeping the usage of fossil fuel. The big storage capacity and common happening of saline aquifers involvement research workers as a possible storage site. An accurate equation of province ( EoS ) is critical in reactive conveyance theoretical accounts that predict the ultimate destiny of stored CO2. A new EoS, for the mixtures of CO2/water/salt fluids, is proposed to be developed utilizing the recent alteration of the statistical association fluid theory ( SAFT ) , to aqueous mixtures of electrolyte solution ( SAFT-VRE ) , [ Galindo, A. et al. , 1999 ] and molecular kineticss simulation ( utilizing DL_POLY plan ) . Besides betterment to thermodynamic description of the fluids mixtures at the critical point is aimed. This is to be achieved by bettering the parameterisation of already bing literature potencies and incorporation of intelligence footings ( 1 ) to the residuary free energy of ion-ion interaction to account for ion-ion scattering consequence and ( 2 ) to the monomer-monomer interaction part via a third-order disturbance in powers of the square-well deepness. Obtained binary interaction parametric quantities ( from sub-binary systems ) will be validated against experiment and/or simulation consequence ( where no experimental consequences are available ) .
2.2 Background: Introduction
Global anthropogenetic emanations of nursery gases ( GHG ) , chiefly CO2, from fossil fuel burning to the ambiance have being identified as impacting the stableness of the earthaa‚¬a„?s clime. A general consensus by the Inter-governmental Panel on Climate Change ( IPCC ) is that the emanations and comparative causes must be mitigated [ IPCC, 2001 ] . Besides, run intoing the United Nations Framework Convention on Climate alteration ( UNFCC ) stabilisation mark, big decreases in GHG emanations is required, peculiarly CO2 emanations. Underground geologic storage of CO2 ( from letter paper emanation beginnings ) is viewed as a feasible economic scheme of accomplishing this decrease every bit good as increasing the flexibleness in developing alternate energy beginnings [ Czernichowski-Lauriol et al. , 2002 ] . CO2 injection in geologic reservoirs is employed by the crude oil industry to better recovery rates of oil and gas in worsening oil and gas Fieldss, a procedure known as enhanced oil recovery ( EOR ) [ NETL, 2010b ] . The big volume of saline aquifers ( 20 % to 500 % of projected CO2 emanations to 2050, Davidson et al. , 2001 ) , common happening and non-potential beginning for drinkable H2O makes storage in saline aquifers an option considered for geologic segregation of CO2. Storage in saline aquifers can be achieved by either physical caparison ( floaty supercritical CO2 ) , solubility caparison ( Dissolution in seawater ) , ionic ( dissolved hydrogen carbonate ion ) and mineral ( solid carbonate precipitate ) pin downing mechanisms [ Czernichowski-Lauriol et al. , 2002 ] . However, disintegration of CO2 in saline Waterss ( solubility pin downing ) is considered the most of import long-run keeping province [ Bickle, M. et al. , 2007 ] . A cardinal facet of CO2 segregation is the demand to accurately foretell CO2 solubility in aqueous solution at high force per unit areas ( associated with deep deepness injection ) , over a geologic period of clip. Hence a dependable equation of province is an indispensable ingredient for conveyance patterning which predicts the ultimate destiny of stored CO2. Our thought is to develop a robust equation of province utilizing the statistical associating fluid theory ( SAFT ) attack plus molecular dynamic simulations ( utilizing DL_POLY plan ) to accurately depict the thermodynamic belongingss and vapour-liquid equilibrium of CO2, H2O and salt mixtures, as this will help in depicting the solubility of CO2 in saline aquifers.
2.3 Background: Statistical Associating Fluid Theory ( SAFT ) .
Carbon dioxide is normally injected into saline aquifers as a supercritical fluid. Interactions between CO2, H2O and salt ( NaCl ) are a combination of tie ining and non-associating concatenation fluid mixtures. While H2O is a extremely tie ining fluid [ Gill-Villegas. A. , et Al. 1997 ] , CO2 is considered a non-associating concatenation molecule and strong electrolyte solution of aqueous Na chloride ( NaCl ) have being considered non-associating [ Robinson, R. A. , 1965 ] . However this lone applies to ambient conditions. It has being shown that ion-ion association ( ion partner offing ) occur in aqueous electrolyte solution as temperatures approach the critical point for H2O ( due to the lessening in dielectric invariable of H2O ) [ Pitzer, K. S. J. , 1993 ] . Readily employed technology equation of province, such as Peng-Robinson, Soave-Redlich-Kwong and Benedict-Webb-Rubin, are betterments on the difficult sphere part and/or average field term of the new wave der Waals equation. Their empirical attack can accurately depict the thermodynamic behavior of simple, nearly-spherical low molecular mass hydrocarbon and simple inorganic ( e.g. N2, CO, O2 etc ) . However the mention quickly becomes inappropriate in foretelling fluids mixtures of extremely tie ining and non-spherical concatenation molecules [ Economou. G.I. , 2002 ] . This is because, for such fluids new intermolecular forces such as columbic forces, complexing forces and forces due to association comes into drama which are non explicitly taking into consideration by such attack [ Economou. G.I. , 2002 ] . A more appropriate mention should integrate the part of molecular form and association as they surely affect the fluid construction. It is in this visible radiation that Chapman et Al. ( in 1990 ) proposed an equation of province for tie ining concatenation molecules based on a series of Wertheim first-order thermodynamic disturbance enlargement of Helmholtz energy [ Wertheim, M. S. , 1984a ; 1984b ; 1986a ; 1986b ; 1986c ] called statistical associating fluid theory ( SAFT ) .
SAFT relates the thermodynamic belongingss of a fluid to its intermolecular forces. In the SAFT attack, the molecular part to the macroscopic behavior of the fluid is via a amount of footings which include the separate consequence of the molecular form ( concatenation length ) , scattering interaction and molecular association [ Galindo, A. et al. , 1998 ] . This consideration makes SAFT suited for a wide scope of molecules, from non-associating near-spherical and non-spherical molecules, to tie ining near-spherical and non-spherical molecules [ Chapman, G. W. et al. , 1990 ] . SAFT basically considers complex molecules to be built-up of tangentially touching spherical monomers. The general equation for SAFT Helmholtz free energy for tie ining concatenation molecules is given by
( 1 )
Where is the ideal free energy, is the extra Helmholtz energy of the free monomers, is the Helmholtz free energy alteration on linking the monomers into ironss and is the part to the free energy due to intermolecular association. are the figure of molecules, Boltzmann invariable and temperature ( K ) severally [ Gill-Villegas, A. et al. , 1997 ] . Several alteration of the original Lennard-Jones ( LJ ) section ( used by Chapman et al. ) has being made to better the description of the monomer-monomer part [ Banaszak, M. et al. , 1993 ; Ghonasgi, D. & A ; Chapman, G. J. , 1994 ; Tavares, F. W. et al. , 1995 ] .
In the modified version of SAFT ( SAFT-VR ) developed by Gill-Villegas and colleagues, an arbitrary potency of variable scope is used to used to depict the concatenation molecules of hard-core section. An extra derived parametric quantity, the scope ( , allows for intervention of extremely non-conformal fluid mixtures [ Gill-Villegas, A. et al. , 1997 ] . In the SAFT-VR attack, the monomer-monomer scattering interactions are represented by second-order high-temperature disturbance enlargement utilizing a compact look for the first order disturbance term, ( average attractive energy ) . The derived second-perturbation term, describes fluctuation of the attractive energy due to the fluid compaction consequence of [ Gill-Villegas, A. et al. , 1997 ] . This consequence correlates to macroscopic thermodynamic compaction described by local denseness fluctuation of the fluid. The look is derived from Barker and Henderson disturbance theory, and given as
( 2 )
Where and are the first and 2nd disturbance term associated with the attractive energy of the variable scope. is the Helmholtz free energy for a mixture of difficult domains. is the opposite of temperature ( T in Kelvin ) , and is Boltzmann invariable. When applied to mixtures SAFT-VR is merely straightforward. The average value theorem ( MVT ) for pure constituent is still applicable in measuring for monomer mixtures. Besides the contact value and radial distribution map for pure constituents in mixtures can be combined obtaining similar looks to that of pure constituent [ Gill-Villegas, A. et al. , 1997 ] . The equation was tested for a square good possible ( SW ) , a Yukawa ( Y ) potency and a Sutherland ( S ) potency. Excellent representation of the vapour-liquid equilibrium ( VLE ) for binary mixtures of H2O with non-electrolytes was observed below the critical part, when vapour-liquid coexistence densenesss were correlated with fake consequences. SAFT-VR was nevertheless unequal in depicting the thermodynamic behavior at the critical part. Galindo, A. et al. , 1998, applied several commixture regulation to account for the binary mixtures of non-conformal fluids utilizing SAFT-VR, but the attack failed to adequately depict stage behavior at the critical part.
Our purpose is to widen the order of thermodynamic disturbance of the monomer-monomer attractive energy term to depict the stage behavior at the critical part.
Having obtained a good description of the thermodynamic belongingss of H2O, Galindo, A. et al. , 1999, extended the SAFT attack to mixtures of strong electrolyte solution ( SAFT-VRE ) utilizing SW potency. Here, H2O molecules are modelled as difficult sphere with four tie ining short scope sites depicting the hydrogen-bonding association and electrolyte molecules are modelled as two difficult domains ( cation and anion ) of different sizes. The mean-spherical-approximation ( MSA ) for the restricted crude theoretical account was used to account for the long-range columbic ion-ion interaction. The long scope water-water and ion-water attractive interaction were modelled as second-order high temperature disturbance enlargement as with the SAFT-VR attack [ Galindo, A. et al. , 1999 ] . The general look for the SAFT-VRE attack takes into consideration parts from the ion-ion interactions and is given as
( 3 )
Here is the part to the Helmholtz free energy from ion-ion interactions. All other footings are the same as those in equation ( 1 ) . The SAFT-VRE attack can be easy extended to solutions of assorted salts as the possible parametric quantities used are determined in footings of ions. For all studied salts ( including NaCl ) in a temperature scope of 273-373K, the SAFT-VRE calculated vapour force per unit area reproduced the experimental information well. However, saturated liquid densenesss are somewhat overestimated [ Galindo, A. et al. , 1999 ] .
Our purpose is to better the VLE anticipation over an increased temperature scope by the add-on of new footings to account for the ion-ion scattering interaction consequence. We will run the DL_POLY molecular kineticss simulation bundle on one and two constituent systems utilizing the literature possible parametric quantities for pure CO2 molecules and aqueous NaCL salt solution mixtures severally and validate consequence against experiment. Once validated, simulation consequences can be used in add-on to experimental informations to formalize the SAFT-VR looks. We will besides farther run SAFT programme for CO2 as non-associating concatenation molecule utilizing equation ( 1 ) and SAFT-VRE for mixtures of tie ining H2O molecule and two ( cation and anion ) tie ining ions utilizing equations ( 3 ) .
2.4 Research Hypothesis and Aims
We propose developing a robust thermodynamic equation of province for CO2/water/salt mixtures by utilizing the SAFT attack and Molecular kineticss ( DL_POLY ) simulations. This entails bettering the parameterisation of the theoretical account and including new footings to better the thermodynamic descriptions at the critical point of the mixtures. The undertaking takes advantage of the recent progresss in patterning extremely non-conformal tie ining concatenation mixtures [ Gill-Villegas, A. et al. , 1997 ] , strong electrolytic mixtures [ Galindo, A. et al. , 1999 ] and simulation representation of complex fluid mixtures [ Koneshan, S. et al. , 2000 ; Lopez-Rendon, R. et al. , 2008 ] , doing it executable. Never before has the quaternate system of CO2/water/salt mixtures being modelled utilizing a SAFT attack and the increasing demand to extenuate GHG emanations ( particularly CO2 ) makes the undertaking rather seasonably. The hypothesis and aims of the assorted work bundles ( WP ) are:
WP1: the accurate anticipation of the thermodynamics and structural belongingss of CO2 and aqueous solution mixtures will depend on the interaction possible theoretical account. Our aims are to formalize molecular dynamic simulation consequences ( DL_POLY ) for the assorted constituent mixtures utilising literature potencies validated against experiment. Where good lucifers are obtained, this will function as a mention to better the quality of SAFT parametric quantities ( where experimental informations are missing ) .
WP2: the average spherical estimate ( MSA ) used for the restricted crude theoretical account ( RPM ) in the SAFT-VRE attack histories entirely for the ion-ion coulombic interaction for which electrolyte ions are assumed to be immersed in a unvarying dielectric medium. Diffusing interactions between the ions, nevertheless, are missing. Our aim is to include a new term to account for the ion-ion scattering consequence in the salt solution.
WP3: in the SAFT-VR attack the monomer belongingss are obtained from a second-order high temperature disturbance enlargement of the SW variable scope potency. Our aim is to add a third-order disturbance term in powers of the square good depth ( ) of to better the description of the VLE at the critical point.
2.5 Programme and Methodology
WP1: we will first run DL_POLY with one constituent system for pure H2O and CO2 molecules utilizing literature interaction potencies [ Lopez-Rendon, R. et al. , 2008 ] at a given temperature and force per unit area scope. Water will be represented as an drawn-out simple point charge ( SPC/E ) , as this theoretical account takes into history the polarisation of H2O in an approximative manner. Simulated consequences will be validated against experimental consequence to verify the serviceability of the chosen force field. Validated consequences for the pure constituents of CO2 and H2O will be used to run DL_POLY with binary mixture system for water-CO2, water-NaCL and water-water mixtures, utilising literature interaction potency for NaCl [ Koneshan, S. et al. , 2000 ] . Simulated consequences will once more be tested against experimental consequences. If convergences occur between the vapour force per unit area and liquid densenesss the fake consequence will function as a mention to correlate the SAFT theoretical account. A molecular simulation tracks the gesture of single molecules in clip and can be used to construe experimental consequences or serve as replacement where no experimental informations are available.
Finally, we will run SAFT with one constituent and with binary mixture system for pure molecules of H2O and CO2 severally, utilizing literature parametric quantities for the SAFT-VR attack ( i.e. square good scope, , deepness, and section diameter, , association energy, and association volume, ) [ Galindo, A. et al. , 1999 ; Alain, V. et al. , 2004 ] . Where and, are the association energy and volume due to short scope attractive force between a H site and O negatron on two different molecules. Water is modelled as a hard-sphere with four short-range non-central associating sites stand foring H bonding ( ) while CO2 is modelled as digressive touching spherical dimer concatenation molecule ( without tie ining sites, ) . These parametric quantities are validated against experimental and/or simulation consequence and optimised if non well-fitted utilizing the simplex method. This is of import in depicting the thermodynamic belongingss of existent substances.
WP2: we will utilize the optimized intermolecular potencies for pure H2O constituent ( from WP1 ) to run SAFT with treble mixture system for mixtures of H2O in strong electrolytic solution of NaCl, utilised the drawn-out version of SAFT-VR for strong electrolyte solution ( SAFT-VRE ) [ Galindo, A. et al. , 1999 ] . Water ( dissolver ) is modelled in the same spirit as SAFT-VR and parts to Helmholtz free energy are given by equation ( 3 ) . Solvent-solvent, solvent-ion and ion-ion interaction part will be considered [ Galindo, A. et al. , 1999 ] . MSA assume RPM will account for the long-range coulombic interaction. However the premise of a nothing long scope attractive square good ion-ion interaction will be relaxed. This has being assumed in antecedently modelled water-NaCl mixtures [ Galindo, A. et al. , 1999 ; Gill-Villegas, A. et al. , 2000 ] for which equally-sized Attic molecules are solvated in a unvarying dielectric dissolver medium at ambient conditions. This attack has nevertheless failed to depict, accurately, thermodynamic belongingss at the critical point. We will be taking into history the molal concentration of saline aquifers to loosen up this premise and add a new term accounting for the ion-ion scattering consequence of the coulombic part to Helmholtz free energy. This is so because the consequence of strong electrolytic ions on belongingss of extremely tie ining polar dissolver such as H2O can change the critical invariable of H2O within the critical point, taking to ion-ion association [ Koneshan, S. et al. , 2000 ] . NaCl intermolecular parametric quantities will be taking from literature [ Galindo, A. et al. , 1999 ] .
Finally, determined intermolecular parametric quantities of the treble mixture will be fitted against experimental and/or simulated consequence and optimised utilizing the simplex method.
WP3: in conclusion, in the SAFT-VR attack ( footing for formalism in the SAFT-VRE ) , part to the Helmholtz free energy due to long-range scattering forces is obtained via a 2nd order high-temperature disturbance enlargement of the variable scope [ Gill-Villegas, A. et al. , 1997 ] . This degree of estimate has magnificently described thermodynamic behaviors below the critical point but fails as temperatures approach the critical point. It has being suggested that integrating a new term, due to a 3rd order disturbance in the powers of the attractive square-well deepness ( ) , to the monomer-monomer section part will significantly better thermodynamic description at the critical point ( recent personal communicating of Masters with Galindo ) . This we would measure for binary mixtures of optimised treble intermolecular SW possible parametric quantities of H2O with aqueous NaCl solution ( WP2 ) and antecedently optimised CO2 dimer intermolecular parametric quantities ( WP1 ) . It should be noted that ne’er earlier has CO2/water/salt mixtures being modelled utilizing SAFT attack and so no theoretical consequences are available. However the SAFT-VRE attack allows for such complex mixtures in its formalism utilizing comparatively consecutive forward combinations with commixture regulations [ Galindo, A. et al. , 1999 ] . We will run CO2/water/salt mixtures utilizing SAFT with quaternate system. New interaction to be considered will be CO2-water interactions in coexisting stages. Salt will be restricted to the liquid stage ( as it is assumed to be non-volatile even at high temperatures ) [ Parisod, C. J. , 1981 ] .
Finally, modelled consequences will be compared with experimental consequences. Knowing the Helmholtz free energy all other macroscopic thermodynamic parametric quantities at VLE can be evaluated, hence the solubility of CO2 in saline aquifer determined. Bing able to accurately foretell the solubility of CO2 in saline aquifers is indispensable for long-run segregation of injected CO2.
Milestones of proposed research: these are
M1.1: New simulation consequences utilizing DL_POLY one constituent and binary mixture systems for CO2/water/salt mixtures.
M1.2: Improved parameterisation of literature intermolecular potency for pure fluids of H2O and CO2 utilizing SAFT one constituent and binary mixture system, severally.
M2.1: Reformulation of the ion-ion coulombic interaction part to Helmholtz free energy to integrate a scattering consequence between ions.
M2.2: Improved description of thermodynamic belongingss of H2O at critical point.
M3.1: Alteration of SAFT-VR formalism in the monomer-monomer section part to overall Helmholtz free energy to heighten prognostic capableness of attack within the critical point of mixtures.
M3.2: New intermolecular parametric quantities for CO2/water/salt mixtures utilizing SAFT with quaternate system attack validated against experimental consequences.
M3.3: Better VLE representation of theoretical theoretical account with experiment at the critical point of fluid mixtures.
2.6 Relevance to Academic Beneficiaries
One cardinal benefit gettable from this undertaking is the development of a robust equation of province utilizing statistical mechanics for CO2/water/salt fluid mixtures. Once this betterment is attained faster and more accurate description of the mixture will be developed heightening the anticipation of CO2 solubility in saline H2O utilizing theoretical theoretical accounts. A positive result will surely involvement the research community, and this will correlate straight to industries ( such as the crude oil industries ) acting, or meaning to research the option of, CO2 segregation in saline aquifers. Better anticipation of the thermodynamic belongingss of electrolytic solution at the critical part utilizing a statistical mechanics attack, is one of great involvement to applied physical scientific disciplines, molecular natural philosophies and the technology community.