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# Metal Dusting Calculations

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```Metal Dusting Calculations
Project 138-00
By
Sheldon W. Dean
October 22, 2002
Project 138 Program
вЂў Gas compositions based on industrial
experience
вЂў 7 different compositions
вЂў 2 temperatures, 1100o & 1300oF
вЂў 2 pressures, 1 Atm., 543 psia (36.96
Atm.)
Question?
вЂў How to evaluate severity of conditions?
вЂў Answer depends on mechanism of
metal dusting.
вЂў Most investigators believe that carbon
deposits and enters the metal.
вЂў This leads to several possible measures
of severity
Carbon Activity
вЂў Carbon activity is a thermodynamic
concept.
вЂў Carbon activity is a relative measure of
carbon concentration driving force.
вЂў Carbon activity is based on graphite as
ground state.
вЂў Analogy to relative humidity.
Carbon Activity
вЂў Carbon activity = Ac
вЂў Ac = [C] / [CG]
вЂў Ac = pc / pG
вЂў Ac = 1 for graphite
Carbon Activity in Syngas
вЂў 2 CO п‚® CO2 + C Boudouard
вЂў CO + H2 п‚® H2 + C Hydrogenation
вЂў Two different reactions can deposit
carbon
Carbon Activity in Syngas
вЂў Problem! Which reaction to use?
вЂў Note that the carbon activity increases
as temperature decreases.
вЂў Temperature where Ac = 1 is CPT
(Carbon Precipitation Temperature)
вЂў Therefore, the higher the CPT the larger
the Ac at a fixed temperature.
Calculating Ac
вЂў Residence time of gas in process is
short, a few seconds.
вЂў Residence time of adsorbed gas on
metal surface is long
вЂў So adsorbed phases tend towards
equilibrium
вЂў When equilibrium occurs both reactions
give the same Ac.
Water Gas Reaction
вЂў CO + H2O = H2 + CO2
вЂў When this reaction equilibrates
composition both reactions give the
same Ac.
Methane Formation?
вЂў
вЂў
вЂў
вЂў
вЂў
C + 2H2 п‚® 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
вЂў Carbon formed on metal surface must
diffuse into the matrix.
вЂў Graphite solubility increases with
temperature
вЂў FCC iron assumed for metal. Diffusivity
increases with temperature.
вЂў No metal dusting when Ac < 1.
Diffusion of Carbon in
Metal
вЂў FickвЂ™s Law:
Fc = Dcп‚¶[C]/п‚¶x
Fc = Dc[ (Ac вЂ“ 1) вЂ“ f(x,t)]
Metal Dusting Severity
вЂў If diffusion is a rate limiting step then the
following value is a reasonable measure
of severity:
S = DcXc (Ac вЂ“ 1)
вЂў CPT is also another parameter that can
be used to judge severity.
вЂў Ac may also be used, but note
temperature problem.
CPT Calculation
вЂў Trial and error procedure.
вЂў Start with either 1100 or 1300F
вЂў Calculate water gas equilibrium
composition (Appendix)
вЂў Calculate CPT (Eqn. 5)
вЂў Use that temperature to recalculate
water gas equilibrium composition.
CPT Calculation Cont.
вЂў Recalculate CPT
вЂў Recalculate water gas equilibrium
composition, etc.
вЂў Usually less than three trials were
necessary to obtain agreement to within
1o K
Additional Calculations
вЂў Carbon Activity:
вЂ“ Use water gas equilibrium compositions at 1100o
and 1300oF
вЂ“ Calculate carbon activity using Eqn. 7
вЂў Severity:
S = DcXc (Ac вЂ“ 1)
вЂў Values of Dc and Xc were taken from ref.1
Results of Calculations
вЂў
вЂў
вЂў
вЂў
вЂў
вЂў
[See Table 1 for numerical values]
CPT varies from 990В° to 2095В°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 1300В°F is least severe
Gas #7 at 36.96Atm and 1300В° is most
severe
Conclusions
вЂў The calculated values of Ac, CPT, and S
should be helpful in understanding the results
of Argonne Study.
вЂў Reviewing the damage in the light of the
calculated values may shed light on the
mechanism of attack.
вЂў If agreement is found then this calculation
approach will have merit in assessing
environments.
```
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