Formation of iron carbonyl between a 1/2 pct Mo steel and high-pressure gases containing carbon monoxide

Abstract

An atomic absorption spectrophotometer was used to measure the formation of iron carbonyl between 1/2 pct Mo steel pipes and flowing high-pressure gases (to 6.9 MPa) containing carbon monoxide. The net formation rate, r, of iron carbonyl was measured as a function of the velocity, temperature, and pressure of the gases to determine the conditions that prevent its formation. These variables and the gas composition affected r as follows: 1) Rate r increases linearly with the gas velocity. Under equilibrium conditions, r is proportional to the gas velocity, but it reaches a limiting value above a critical gas velocity when equilibrium is not maintained. 2) Between 44 and 266 °C, r increases with temperature to a sharp maximum at 177°C in a gas mixture containing 17 CO-51 H2-30 CH4-2 CO2 (vol pct). The corresponding maximum in carbon monoxide occurs at 245 °C. The data predict that iron carbonyl formation will be negligible above 270°C in the gas mixture or above 285 °C in carbon monoxide. 3) Rate r depends on Pco in accordance with the equation r = kPn, where n = 4 to 5 under equilibrium conditions and n = 2 off equilibrium. The analytical procedure used to detect ±6 vpb (volume parts per billion) of iron carbonyl and ±1.5 vpb of nickel carbonyl in flowing high pressure gases is described. The reaction rates are described in terms of kinetics and thermodynamic equilibrium.