1. Amrit Kalra, Ray French, Sheila Dubey, Ashok Dewan
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 – , Morristown NJ

2. 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 acid
HCOOH
CH3COOH
CH3CH2COOH
CH3CH2CH2COOH
Highlight:
HAc aq H+ +
Ac-
Kdiss
HAc aq v
KHenry
Use:
Thermodynamics / Chemistry framework
Experimental Physical Properties Data
OLI Stream Analyzer
Dissociation
VLE partitioning

3. VFAs can increase CO2 corrosion rates
X-65 Steel
3 % NaCl
pCO2 = 0.96 bar
pH = 4
T = 25 oC
1000 rpm
Source: Effect of organic acids in CO2 corrosion, Paper No , NACE 2007
Questions?
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?

4. VFA’s influence on TOL and BOL Corrosion
(for TOL and BOL Corrosion)
HYDROCARBON
Top-of-Line (TOL) & Bottom-of-Line (BOL) Corrosion

5. Phase distribution of Acetic acid in Water/Gas/Condensate
VAPOR
WATER
HAc aq H+ +
Ac- v 2 1
Kdiss
KHenry
Kaq, dimer
HYDROCARBON
Ko/w HC
Kv, dimer
Kv, dimer 1
HAc
HAc
VAPOR 2 v 2
KHenry 1
HAc
HAc
HAc 2 HC aq 2
Kaq, dimer
Kdiss
Ko/w
HYDROCARBON
WATER H+ +
Ac-
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.

6. Effect of pH and temperature on Ionic Chemistry of H2S, CO2 and NH3
Effect of pH on CO2, H2S and NH3 speciation
Effect of temperature on CO2 speciation

7. Dissociation of Organic acids (C1-C4) in water
Stream Analyzer
HAc aq H+ +
Ac-
Kdiss
Experimental Data
Dissociation constants of organic acids in aqueous solutions, G. Kortum, W. Vogel, and K. Andrussow, London Butterworths 1961
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]

8. Mi is molality and Pi is partial pressure
Henry’s Law constants of Organic acids (C1-C4)
HAc aq v
KHenry
T = 25 oC
T = 25 oC
KHenry = Mi / Pi
Mi is molality and Pi is partial pressure
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)

9. Thermodynamic (non) co-operation
KH True
KH Apparent
HAc
HAc v v
Stream Analyzer KH KH
KH, Molar atm-1
HAc
HAc
Ac- + H+ aq aq
Kdiss
Kdiss H+ +
Ac-
From Stream Analyzer:
Acetic acid at pH = 5, 1 atm, 25 oC, HAc + Ac- = 1mM
[HAc]aq = mM [Ac-] = mM
[PHAc]vap = x 10-9 atm
KH True = 5500M/atm KH Apparent = M/atm
Usually, total concentration (dissociated + undissociated) is measured in water analysis
KHenry should be coupled with degree of dissociation

10. Partitioning of Organic acids between Oil and Water
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)
C1 extrapolation: Ko/w =
Linear plot for Ko/w as a function of carbon number, for organic acids.
All four acids (C1-C4) should not be lumped together.
Simulations done in Stream Analyzer for model HC’s.
Ko/w is weak function of temperature.

11. Phase distribution of Acetic acid in Water/Gas/Condensate
VAPOR
WATER
HAc aq H+ +
Ac- v 2 1
Kdiss
KHenry
Kaq, dimer
HYDROCARBON
Ko/w HC
Kv, dimer
Dimerization can be ignored for
OA in Water phase < 10 %
OA in gas phase < 1 %
Engineering data and Thermodynamic Framework for four organic acids (C1-C4) is obtained for different operating conditions

12. Acetic Acid: Comparison of MSE and H+ models
Dissociation of Acetic acid
Not significant difference in the value of pKd
Henry’s Law Constant MSE : M atm-1
(25 oC and 1 atm) H+ : M atm-1

13. Conclusions
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

14. Questions??