Mass composition of iodine (%)

Thermodynamics modeling of the

HIx part of the Iodine – Sulfur

thermocycle

M. K. Hadj-Kali, V. Gerbaud, P. Floquet, X. Joulia J.M. Borgard, P. Carles, G. Moutiers November 4 -9, 2007

Salt Palace Convention Center Salt Lake City, Utah

H₂SO₄ Decomposition H₂SO₄ Concentration HI Decomposition HI Vaporization Bunsen Reaction

H

₂

O

H

₂

O

₂ 2 HI ↔↔↔↔ H₂ + I₂ [330 °C] I₂ + SO₂ + 2 H₂O →→→→ H₂SO₄ + 2 HI SO₂ + H₂O + ½ O₂ ↑↑↑↑ [850 °C] H₂SO₄

I

₂

SO

₂

Multiphase behavior (vapor – liquid – liquid – solid),

H

2

O – I

2

: highly immiscible liquid – liquid equilibrium system,

HI – I

2

: solid – liquid equilibrium system,

H

2

O – HI

 Maximum boiling / low pressure azeotrope,  Strong electrolytic system,

 Liquid – liquid equilibrium,

The ternary mixture has two separate liquid – liquid regions,

Solvation reactions occur as well as poly-iodides formation in the solution.

Thermodynamics challenges of HIx system

“ …The sulfuric acid decomposition section of this process can be simulated accurately, but other sections (acid generation and hydrogen iodide decomposition) illustrate the difficulty of modeling phase

behavior, particularly liquid-phase immiscibility, in complex electrolyte systems.”

(Mathias 2005, Fluid Phase Equilibria)

Outline

Iodine-Suldure thermochemical cycle for hydrogen production

HIx modeling Challenges

Proposed model and identification methodology

Validation vs experimental literature data

UNIQUAC Homogeneous model φφφφ-φφφφ

Equation of State PR Boston Mathias

Mixture rules

Complex

Standard _{Symmetric convention}γγγγModel

Electrolytes MHV2

HIx modeling

Engels Solvation Non- Electrolytes Heterogeneous model γγγγ-φφφφ liquid Phase Ideal gas γγγγModel Asymmetric convention NRTL + Pitzer vapor Phase

Equation of State γγγγModel

Symmetric convention Electrolytes

Engels Solvation Wilson

NRTL Neumann Electrolytes

Neumann : Engels + NRTL Mathias et al. : electrolyte

Literature models:

Non- Electrolytes

UNIQUAC Solvation (UQSolv)

New models:

Advantages of the model

φ

-

φ

approach:

 No extrapolation of the vapor pressure law above critical points

 Correct handling of mixture phase behavior above HI critical point (T_C=423K)

Equation of state: Peng Robinson w/ Boston Mathias

α

function

 Accurate behavior of pure components above critical points

Mixing rule: MHV2 including non ideal liquid phase behavior via a G

ex

_model

Activity coefficient model: Engels solvation

 Electrolytic model but with symmetric convention so as to span the whole composition range  Introduces solvation complexes with several solvents

Activity coefficient model: UNIQUAC vs NRTL:

 Combinatorial term handles steric effect

Model parameter identification procedure

Three binary mixtures

 H₂O – I₂: only UNIQUAC binary interaction parameters  HI – I₂: only UNIQUAC binary interaction parameters  H₂O – HI:

• HI solvation by H₂O → equilibrium constant and solvation number m H₂O + HI ↔ 2C

• UNIQUAC binary interaction parameters for H₂O, HI and the solvation complex C

Ternary mixture

Available experimental data

Haase (1963) 21 P = f(T,x,y) VLE LLE VLE LSE LSE LLE LLE ∆H VLE VLE

Data

Neumann (1986) 280 P = f(T,x)

H

₂

O – HI – I

₂

Vanderzee & Gier (1974) 13 Norman (1984) 12 Wüster (1979) 80 P = f(T,x) Pascal (1926) 38 T = f(P,x,y)

H

₂

O – HI

O’keefe & Norman (1982)

5 T = f(x)

HI – I

₂ Haase (1963) / Norman (1985) 6 T = f(x) Kracek (1932) 10 T = f(x) Kracek (1932) 10 T = f(x)

H

₂

O – I

₂

Data sources

Data number

Model prediction of iodine solubility in aqueous solution

H

₂

O – I

₂

identification results

Iodine melting point Neumann Model (w_I2 > 100)

HI – I

₂

identification results

0 20 40 60 80 100 0 20 40 60 80 100 120 Temperature (°C) M a s s c o m p o s it io n o f io d in e ( % )

exp (O'Keefe et Norman, 1982) Ideal UNIQUAC

Neumann UQSolv

Good agreement up to the azeotrope

H

₂

O – HI identification results 1/2

azeotrope 0,1 1 10 100 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 HI mass fraction P re s s u re ( b a r) T=90°C T=130°C T=170°C T=190°C T=230°C Wüster Correlations (1979)

H

₂

O – HI identification results 2/2

H20 - HI liquid - vapor equilibrium curve at atmospheric pressure

-60 -40 -20 0 20 40 60 80 100 120 140 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 HI molar fraction T e m p e ra tu re ( ° C )

exp bubble points (Pascal, 1926) exp dew points (Pascal, 1926) UQSolv bubble curve

UQSolv dew curve

Engels experimental data (0,017 < x_HI < 0,159)

Ternary system H

₂

O – HI – I

₂

Liquid – liquid equilibrium data (Norman, 1984)

HI I2

H2O

6.50 62.24 9.53 3.73 44.83 8.18 UQSolv Neumann criterion Max error (%) Average error (%) 2

∑

_       ₋ = Np exp cal exp P P P Criterion

Liquid – liquid equilibrium (H2O – I2)

Solid – liquid equilibrium (HI – I2)

Vapor – liquid equilibrium (H2O – HI) with solvation (m H2O + HI  2C)

Keeping identified parameters for :

Conclusion and perspectives

HIx non ideal and electrolytic system is modeled by an equation of state (EOS)

approach with complex mixing rules including G

EX

_models.

The EOS approach is well suited with expected process T and P conditions that may be

higher than the HI critical point.

The liquid phase non ideal behavior is modelled combining Engels solvation equations

suitable for any electrolyte composition and activity coefficient UNIQUAC equation.

The resulting UQsolv model predict accurately most of the experimental data available

and accurate, incl. LLE, LSE and LLVE for each binary and ternary system.

Largest discrepancy is found for high HI concentration mixtures where experimental

data is lacking or is uncertain.

Improvements are under investigation:



Polyiodide ion formation