Thermodynamic and transport properties of fluids: towards a LJ-SAFT molecular model valid for n-alkanes

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Thermodynamic and transport properties of fluids:

towards a LJ-SAFT molecular model valid for n-alkanes

Guillaume Galliero, C. Boned, Stephanie Delage Santacreu, Marc Odunlami, F. Montel

To cite this version:

Guillaume Galliero, C. Boned, Stephanie Delage Santacreu, Marc Odunlami, F. Montel. Thermody- namic and transport properties of fluids: towards a LJ-SAFT molecular model valid for n-alkanes.

SAFT2011 discussion meeting. Pau, France, octobre 2011, 2011, pau, France. 2011. �hal-01051603�

THERMODYNAMIC AND TRANSPORT PROPERTIES OF FLUIDS:

TOWARDS A LJ-SAFT MOLECULAR MODEL VALID FOR N-ALKANES

MODELS AND THEORY

We study the limitations of the Lennard-Jones Chain fluid model (LJ-SAFT) to describe simultaneously all thermophysical properties (equilibrium and transport) of some n-alkanes along the vapor-liquid equilibrium line

Molecules movements numerically computed (Newton equations) Spatial and temporal averages (∼10

particles, ∼10 ns)

MOLECULAR DYNAMICS

Guillaume GALLIÉRO

, Christian BONED

, Stéphanie DELAGE

, Marc ODUNLAMI

and François MONTEL

1

Laboratory of Complex Fluids and their Reservoirs, UMR 5150 TOTAL-CNRS, Pau University, FRANCE.

2

Laboratory of Applied Mathematics, UMR 5142 CNRS, Pau University, FRANCE.

3

TOTAL E&P, CST, Avenue Larribau, Pau, FRANCE.

LJ-SAFT

LJC LJ chain

A = A + A

No H-bonding, no polar and flexible

A^LJusing the reference term of Kolafa & Nezbeda, (FPE 1994)

RESULTS

Exact thermophysical properties for a given molecular model

VAPOR-LIQUID EQUILIBRIUM SHEAR VISCOSITY

number of segments: N

+ internal bonds

Freely jointed Lennard-Jones spheres (Lennard-Jones Chain)

1 2 6

L J

4

U r r

σ σ

ε ^{     }

=        −       

FLUID MODEL

( T , )

( ) T

( T , )

η ρ = η + η ρ

Dilute gas Residual/Excess viscosity

VISCOSITY MODEL OF LJC

Extended Chapman for η

and Rousse approximation for η

Galliero and Boned, PRE, 2009, PRE, 2010

Internal rigidity is included by U

= k ε θ ( − θ

)

600 700

500 600

ρ (kg.m^-3)

0 200 400 600

T(K)

200 250 300 350 400 450

ρ (kg.m^-3)

0 100 200 300 400

T(K)

100 120 140 160 180 200

NIST Data MD LJ-SAFT

methane n-butane

N=1 for C1, N=2 for C4, N=3 for C7 and N=4 for C10, σand εhave been adjusted

A^chainusing the chain term of Johnson et al. (JPC 1994)

Accurate apart close to the critical point

Valid for gas, liquid and supercritical states Based on MD results

(10-6Pa.s) 50 100 200 500 1000

(10-6Pa.s) 50 100 200 500 1000 T (K)

100 120 140 160 180 200

η (10-6Pa.s) 5 10 50 100

20 200

NIST Data LJC Corr.

T (K)

200 250 300 350 400 450

η (10-6Pa.s) 5 10 50 100

20 200 500

n-heptane n-decane

methane n-butane

SAFT 2011 Discussion Meeting, October 24-25, Pau, 2011

As well known this fluid model is correct for n-alkanes

Internal degrees of freedom (such as rigidity) should be included in a SAFT like approach

ρ (kg.m^-3)

0 200 400 600

T(K)

300 400 500

ρ (kg.m^-3)

0 200 400 600

T(K)

300 400

n-heptane n-decane

but TcSAFT> TcMD> Tc (↗↗↗↗with N)

T (K)

250 300 350 400 450 500 550

η (10

5 10 50 20

T (K)

300 400 500 600

η (10

5 10 50 20

SURFACE TENSION

T (K)

100 200 300 400 500 600

γ (10-3 N.m-1)

0 5 10 15 20 25 30

NIST Data MD

Crossover approach is needed to deal with the critical region

Surface tensions are slightly overestimated A simple correction of the

critical temperature is sufficient The coupling between DGT/DFT and LJ-SAFT

yields good results

Galliero et al., JCP, 2008

η η η

η is well estimated, except at low liquid T when N ↗ ↗ ↗ ↗ This weakness is related to the flexibility of the chain

Temperature (K)

200 250 300 350 400 450

Viscosity (µPa.s)

0 500 1000 1500 2000

NIST Data k = 0 k = 1 k = 10 k = 100 k = 3000

η

is highly affected by the rigidity while thermodynamic

(fluid phase) only weakly

This degree of freedom (rigidity) can be used to improve simply

the fluid model

Concerning thermal conductivity, λ

is out of range when N ↗

(internal degrees of freedom) but the λ

of LJC is correct