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.