(non)co-operation between Phase Equilibria and Ionic Dissociation

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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
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
HCOOH
CH3COOH
CH3CH2COOH
CH3CH2CH2COOH
Formic acid
Acetic acid
Propionic Acid
Butyric acid
Highlight:
HAc
Use:
aq
Kdiss
H+ + AcDissociation
HAc
KHenry
v
HAc
aq
•
Thermodynamics / Chemistry
framework
•
Experimental Physical Properties Data
•
OLI Stream Analyzer
VLE partitioning
2
VFAs can increase CO2 corrosion rates
X-65 Steel
3 % NaCl
pCO2 = 0.96 bar
pH = 4
T = 25 oC
1000 rpm
Questions?
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?
3
VFA’s influence on TOL and BOL Corrosion
VFAs
(for TOL and BOL
Corrosion)
HYDROCARBON
Top-of-Line (TOL) & Bottom-of-Line (BOL) Corrosion
4
Phase distribution of Acetic acid in Water/Gas/Condensate
Kv, dimer
HAc
VAPOR
1
2
v
HAc
2
KHenry
HAc
HAc
HC
HC
Ko/w
HYDROCARBON
aq
Kdiss
H+ + Ac-
Kaq, dimer
1
2
HAc
2
WATER
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.
5
Effect of pH and temperature on Ionic Chemistry of H2S,
CO2 and NH3
1.0
0.9
Relative fraction
0.8
CO32HCO3H2CO3
NH3
NH4+
S2HSH2S
0.7
0.6
0.5
0.4
0.3
0.2
Effect of pH on CO2,
H2S and NH3 speciation
0.1
0.0
3
4
5
6
7
8
9
10
11
12
13
14
pH
Effect of temperature on
CO2 speciation
6
Dissociation of Organic acids (C1-C4) in water
Stream Analyzer
HAc
Experimental Data
aq
Kdiss
H+ + Ac-
• 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
Dissociation constants of organic acids in aqueous
solutions, G. Kortum, W. Vogel, and K. Andrussow,
London Butterworths 1961
• Temperature effect on Kd: factor
of 5 [0 to 200oC]
• Pressure effects are relatively
small: factor of 1.04 [1 to 100 atm]
7
Henry’s Law constants of Organic acids (C1-C4)
HAc
KHenry
v
T = 25 oC
T = 25 oC
HAc
aq
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)
8
Thermodynamic (non) co-operation
KH Apparent
HAc
HAc
v
KH
v
Stream Analyzer
KH
HAc
HAc
aq
aq
Ac- + H+
Kdiss
Kdiss
KH, Molar atm-1
KH True
H+ + AcFrom 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
9
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.
• Simulations done in Stream Analyzer for
• All four acids (C1-C4) should not be lumped together.
• Ko/w is weak function of temperature.
model HC’s.
10
Phase distribution of Acetic acid in Water/Gas/Condensate
Kv, dimer
HAc
VAPOR
1
2
v
HAc
Dimerization can be ignored for
2
OA in Water phase < 10 %
KHenry
HAc
OA in gas phase < 1 %
HAc
HC
Ko/w
HYDROCARBON
aq
Kdiss
H+ + Ac-
Kaq, dimer
1
2
HAc
2
WATER
Engineering data and Thermodynamic Framework for four organic acids
(C1-C4) is obtained for different operating conditions
11
Acetic Acid: Comparison of MSE and H+ models
0.4
0.35
0.3
0.25
Frac_ (MSE)
0.2
frac (H+)
0.15
• Dissociation of Acetic acid
• Not significant difference in the
value of pKd
0.1
0.05
4.19
3.91
3.79
3.71
3.66
3.61
3.57
3.54
3.51
3.49
3.46
3.44
3.43
3.41
3.39
0
Henry’s Law Constant
MSE : 63.21 M atm-1
(25 oC and 1 atm)
H+ :
92.57 M atm-1
12
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
13
Questions??
14
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