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Volatile Fatty Acids (VFAs)

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

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Volatile Fatty Acids (VFAs)

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  1. 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

  2. HAc aq Kdiss HAc v KHenry H+ + Ac- HAc aq 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 CH3CH2COOH CH3CH2CH2COOH HCOOH CH3COOH Highlight: • 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. 07319, 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. VFAs (for TOL and BOL Corrosion) VFA’s influence on TOL and BOL Corrosion HYDROCARBON Top-of-Line (TOL) & Bottom-of-Line (BOL) Corrosion

  5. Kv, dimer 1 HAc HAc VAPOR 2 2 v KHenry 1 HAc HAc HAc 2 2 aq HC Kaq, dimer Kdiss Ko/w HYDROCARBON WATER H+ + Ac- Phase distribution of Acetic acid in Water/Gas/Condensate Kv, dimer 1 HAc HAc VAPOR 2 2 v KHenry 1 HAc HAc HAc 2 2 aq HC 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. HAc aq Kdiss H+ + Ac- Dissociation of Organic acids (C1-C4) in water Stream Analyzer 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. HAc v KHenry HAc aq Henry’s Law constants of Organic acids (C1-C4) T = 25 oC T = 25 oC KHenry = Mi / Pi Mi is molality and Pi is partial pressure • 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) Experimental Data

  9. Thermodynamic (non) co-operation KH True KH Apparent HAc HAc v v Stream Analyzer KH KH KH, Molar atm-1 HAc Ac- HAc + H+ aq aq Kdiss Kdiss H+ + Ac- From Stream Analyzer: 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 • 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 = 0.0035 • 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. Kv, dimer Dimerization can be ignored for OA in Water phase < 10 % OA in gas phase < 1 % 1 HAc HAc VAPOR 2 2 v KHenry 1 HAc HAc HAc 2 2 aq HC Kaq, dimer Kdiss Ko/w HYDROCARBON WATER H+ + Ac- Phase distribution of Acetic acid in Water/Gas/Condensate 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 : 63.21 M atm-1 (25 oC and 1 atm) H+ : 92.57 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??

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