Thermodynamic (non)co-operation between Phase Equilibria and Ionic Dissociation of Organic acids in

Water/Condensate/Gas systems

Amrit Kalra, Ray French, Sheila Dubey, Ashok Dewan

Shell Global Solutions (US) Inc.3333 Highway 6 South, Houston TX 77082

Presentation at OLI User Conference, October 23 – 24 2007, Morristown NJ

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Volatile Fatty Acids (VFAs)Volatile Fatty Acids (VFAs)Source: 1) Can be naturally found in oil field waters 2) Acid stimulation of wells

Volatile Fatty acids: Low molecular weight carboxylic acids

Formic acid Acetic acid Propionic Acid Butyric acidHCOOH CH3COOH CH3CH2COOH CH3CH2CH2COOH

HAcaq

H+ + Ac-

Kdiss

HAcaq

HAcvKHenry

Dissociation

VLE partitioning

Use:• Thermodynamics /

Chemistry framework• Experimental Physical

Properties Data• OLI Stream Analyzer

Highlight:

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VFAs can increase COVFAs can increase CO22 corrosion rates corrosion rates

Source: Effect of organic acids in CO2 corrosion, Paper No. 07319, NACE 2007

• Thermodynamics/Phase Behavior of organic acids?• Electrochemical properties of organic acids?• Mechanism of CO2 corrosion in presence of VFAs?• Effect of VFAs on formation and protectiveness of Iron Carbonate scale?

Questions?

X-65 Steel3 % NaClpCO2 = 0.96 barpH = 4T = 25 oC1000 rpm

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Top-of-Line (TOL) & Bottom-of-Line (BOL) Corrosion

VFA’s influence on TOL and BOL CorrosionVFA’s influence on TOL and BOL Corrosion

VFAs(for TOL and

BOL Corrosion)

HYDROCARBON

5

VAPOR

WATER

HAcaq

H+ + Ac-

HAcv

HAc22

1

Kdiss

KHenry

Kaq, dimer

HYDROCARBON

Ko/w

HAcHC

Kv, dimer HAc122

Phase distribution of Acetic acid in Water/Gas/CondensatePhase distribution of Acetic acid in Water/Gas/Condensate

Main Issue: Phase distribution is more complicated than simple Henry’s Law when the species can ionize and dimerize.

Underlying principle: Chemical potential of species is equal in all phases.

VAPOR

WATER

HAcaq

H+ + Ac-

HAcv

HAc22

1

Kdiss

KHenry

Kaq, dimer

HYDROCARBON

Ko/w

HAcHC

Kv, dimer HAc122

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Effect of pH and temperature on Ionic Chemistry of HEffect of pH and temperature on Ionic Chemistry of H22S, S, COCO22 and NH and NH33

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

3 4 5 6 7 8 9 10 11 12 13 14pH

Rel

ativ

e fr

actio

n

CO32-HCO3-H2CO3NH3NH4+S2-HS-H2S

Effect of pH on CO2, H2S and NH3 speciation

Effect of temperature on CO2 speciation

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HAcaq

H+ + Ac-

Kdiss

Dissociation of Organic acids (C1-C4) in waterDissociation of Organic acids (C1-C4) in water

• pKd of Formic acid is one pH unit less than (C2-C4) acids.

• Not recommended to lump them all together for Corrosion and Phase Equilibria calculations

• Temperature effect on Kd: factor of 5 [0 to 200oC]

• Pressure effects are relatively small: factor of 1.04 [1 to 100 atm]

Stream Analyzer

Experimental Data

Dissociation constants of organic acids in aqueous solutions, G. Kortum, W. Vogel, and K. Andrussow, London Butterworths 1961

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Henry’s Law constants of Organic acids (C1-C4)Henry’s Law constants of Organic acids (C1-C4)

HAcaq

HAcvKHenry

KHenry = Mi / Pi

Experimental Data

• Lack of good literature data on temperature dependent KH

• Assuming Infinite Dilution Activity Coefficient (IDAC) independent of temperature, vapor pressures of pure components are instead used to provide T dependence (Refer: Sep 2007 Chemical Engineering Progress, Avoid Common Pitfalls When Using Henry's Law, F.L. Smith and A.H. Harvey)

Mi is molality and Pi is partial pressure

T = 25 oC

T = 25 oC

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Thermodynamic (non) co-operationThermodynamic (non) co-operation

HAcaq

H+ + Ac-

HAcv

Kdiss

KH

KH True

HAcaq +Ac-

HAcv

Kdiss

KH

KH Apparent

H+

KH, M

olar

atm

-1

• Usually, total concentration (dissociated + undissociated) is measured in water analysis

• KHenry should be coupled with degree of dissociation

Acetic acid at pH = 5, 1 atm, 25 oC, HAc + Ac- = 1mM

[HAc]aq = 0.054 mM [Ac-] = 0.946 mM

[PHAc]vap = 9.79 x 10-9 atm

KH True = 5500M/atm KH Apparent = 100000 M/atm

Stream Analyzer

From Stream Analyzer:

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Partitioning of Organic acids between Oil and WaterPartitioning of Organic acids between Oil and Water

• Linear plot for Ko/w as a function of carbon number, for organic acids.

• All four acids (C1-C4) should not be lumped together.

C1 extrapolation: Ko/w = 0.0035

• Simulations done in Stream Analyzer for model HC’s.

• Ko/w is weak function of temperature.

M.A. Reinsel, J.J. Borkowski and J.T. Sears, Partition Coefficients for Acetic, Propionic, and Butyric Acids in a Crude Oil/Water System J. Chem. Eng. Data 39 (1994)

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Phase distribution of Acetic acid in Water/Gas/CondensatePhase distribution of Acetic acid in Water/Gas/Condensate

VAPOR

WATER

HAcaq

H+ + Ac-

HAcv

HAc22

1

Kdiss

KHenry

Kaq, dimer

HYDROCARBON

Ko/w

HAcHC

Kv, dimer HAc122

Engineering data and Thermodynamic Framework for four organic acids (C1-C4) is obtained for different operating conditions

Dimerization can be ignored for

OA in Water phase < 10 % OA in gas phase < 1 %

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.43.

39

3.41

3.43

3.44

3.46

3.49

3.51

3.54

3.57

3.61

3.66

3.71

3.79

3.91

4.19

Frac_ (MSE)frac (H+)

Acetic Acid: Comparison of MSE and HAcetic Acid: Comparison of MSE and H++ models models

• Dissociation of Acetic acid

• Not significant difference in the value of pKd

Henry’s Law Constant MSE : 63.21 M atm-1

(25 oC and 1 atm) H+ : 92.57 M atm-1

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ConclusionsConclusions

• Determination of “Free” organic acids is the key in Corrosion studies; correct thermodynamic model is essential• Organic acids show competing behavior for dissociation and partitioning into the vapor phase• KH is very temperature sensitive• Good confirmation of experimental data with Stream Analyzer for the H+ model• Need additional data modeling efforts on organic acids for MSE model to represent influence of methanol & glycols

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Questions??