Transcript Metal Dusting Calculations

Metal Dusting Calculations
Project 138-00
By
Sheldon W. Dean
October 22, 2002
Project 138 Program
• Gas compositions based on industrial
experience
• 7 different compositions
• 2 temperatures, 1100o & 1300oF
• 2 pressures, 1 Atm., 543 psia (36.96
Atm.)
Question?
• How to evaluate severity of conditions?
• Answer depends on mechanism of
metal dusting.
• Most investigators believe that carbon
deposits and enters the metal.
• This leads to several possible measures
of severity
Carbon Activity
• Carbon activity is a thermodynamic
concept.
• Carbon activity is a relative measure of
carbon concentration driving force.
• Carbon activity is based on graphite as
ground state.
• Analogy to relative humidity.
Carbon Activity
• Carbon activity = Ac
• Ac = [C] / [CG]
• Ac = pc / pG
• Ac = 1 for graphite
Carbon Activity in Syngas
• 2 CO  CO2 + C Boudouard
• CO + H2  H2 + C Hydrogenation
• Two different reactions can deposit
carbon
Carbon Activity in Syngas
• Problem! Which reaction to use?
• Note that the carbon activity increases
as temperature decreases.
• Temperature where Ac = 1 is CPT
(Carbon Precipitation Temperature)
• Therefore, the higher the CPT the larger
the Ac at a fixed temperature.
Calculating Ac
• Residence time of gas in process is
short, a few seconds.
• Residence time of adsorbed gas on
metal surface is long
• So adsorbed phases tend towards
equilibrium
• When equilibrium occurs both reactions
give the same Ac.
Water Gas Reaction
• CO + H2O = H2 + CO2
• When this reaction equilibrates
composition both reactions give the
same Ac.
Methane Formation?
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C + 2H2  CH4
This reaction is possible, but very slow.
Three molecule reaction.
If this reaction equilibrates, Ac < 1.
I assumed that this reaction does not
occur.
Carbon in Metal
• Carbon formed on metal surface must
diffuse into the matrix.
• Graphite solubility increases with
temperature
• FCC iron assumed for metal. Diffusivity
increases with temperature.
• No metal dusting when Ac < 1.
Diffusion of Carbon in
Metal
• Fick’s Law:
Fc = Dc[C]/x
Fc = Dc[ (Ac – 1) – f(x,t)]
Metal Dusting Severity
• If diffusion is a rate limiting step then the
following value is a reasonable measure
of severity:
S = DcXc (Ac – 1)
• CPT is also another parameter that can
be used to judge severity.
• Ac may also be used, but note
temperature problem.
CPT Calculation
• Trial and error procedure.
• Start with either 1100 or 1300F
• Calculate water gas equilibrium
composition (Appendix)
• Calculate CPT (Eqn. 5)
• Use that temperature to recalculate
water gas equilibrium composition.
CPT Calculation Cont.
• Recalculate CPT
• Recalculate water gas equilibrium
composition, etc.
• Usually less than three trials were
necessary to obtain agreement to within
1o K
Additional Calculations
• Carbon Activity:
– Use water gas equilibrium compositions at 1100o
and 1300oF
– Calculate carbon activity using Eqn. 7
• Severity:
S = DcXc (Ac – 1)
• Values of Dc and Xc were taken from ref.1
Results of Calculations
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[See Table 1 for numerical values]
CPT varies from 990° to 2095°F
Ac varies from 0.041 to 7119
S varies from <0 to 376,144 x10-12 cm2/sec
1 Atm. values always less than 36.96Atm.
Gas #1 at 1Atm and 1300°F is least severe
Gas #7 at 36.96Atm and 1300° is most
severe
Conclusions
• The calculated values of Ac, CPT, and S
should be helpful in understanding the results
of Argonne Study.
• Reviewing the damage in the light of the
calculated values may shed light on the
mechanism of attack.
• If agreement is found then this calculation
approach will have merit in assessing
environments.