Metal Dusting Calculations

1. Metal Dusting Calculations Project 138-00 By Sheldon W. Dean October 22, 2002

2. 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.)

3. 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

4. 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.

5. Carbon Activity • Carbon activity = Ac • Ac = [C] / [CG] • Ac = pc / pG • Ac = 1 for graphite

6. Carbon Activity in Syngas • 2 CO  CO2 + C Boudouard • CO + H2  H2 + C Hydrogenation • Two different reactions can deposit carbon

7. 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.

8. 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.

9. Water Gas Reaction • CO + H2O = H2 + CO2 • When this reaction equilibrates composition both reactions give the same Ac.

10. Methane Formation? • 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.

11. 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.

12. Diffusion of Carbon in Metal • Fick’s Law: Fc = Dc[C]/x Fc = Dc[ (Ac – 1) – f(x,t)]

13. 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.

14. 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.

15. 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

16. 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

17. Results of Calculations [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

18. 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.