volatile fatty acids (vfas)
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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
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
4
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
6
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
7
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??