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

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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, 1100 o & 1300 o F 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 = A c A c = [C] / [C G ] A c = p c / p G A c = 1 for graphite

Carbon Activity in Syngas 2 CO  CO 2 + C Boudouard CO + H 2  H 2 + 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 A c = 1 is CPT (Carbon Precipitation Temperature) Therefore, the higher the CPT the larger the A c at a fixed temperature.

Calculating A c 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 A c.

Water Gas Reaction CO + H 2 O = H 2 + CO 2 When this reaction equilibrates composition both reactions give the same A c.

Methane Formation? C + 2H 2  CH 4 This reaction is possible, but very slow. Three molecule reaction. If this reaction equilibrates, A c < 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 A c < 1.

Diffusion of Carbon in Metal Fick’s Law: F c = D c  [C]/  x F c = D c [ (A c – 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 = D c X c (A c – 1) CPT is also another parameter that can be used to judge severity. A c 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 1 o K

Additional Calculations Carbon Activity: –Use water gas equilibrium compositions at 1100 o and 1300 o F –Calculate carbon activity using Eqn. 7 Severity: S = D c X c (A c – 1) Values of D c and X c were taken from ref.1

Results of Calculations [See Table 1 for numerical values] CPT varies from 990° to 2095°F A c varies from 0.041 to 7119 S varies from <0 to 376,144 x10 -12 cm 2 /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 A c, 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.

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