Ethylene production

THERMODINAMYC ASPECTS

1. OXIDATION

The oxidation phenomenon is considerate as the most important corrosion reaction at high temperature. The metals and alloys oxidize when are exposed to air or environments with high oxygen potential, at elevated temperature with a strong effect of temperature over oxidation rate [Lay, G.Y., 1990].

Thermodynamically, it is probable the formation of an oxide over a metal surface when the oxygen potential in the atmosphere is higher than the partial pressure of oxygen in equilibrium with the oxide. This partial pressure in equilibrium could be determined from the standard free energy change (ΔG) for the oxide formation, considering this reaction:

(1.1)[pic 1]

 (1.2)[pic 2]

Assuming the metal and oxide activities as the unity, we obtained:

 (1.3)[pic 3]

The standard free energy of formation for various oxides as a function of temperature and the relative partial pressures of oxygen in equilibrium with the oxide are summarized in Ellingham/Richardson diagram as shown in Figure 1.1. From this diagram it is possible to determine the potential of oxygen in oxidizing atmospheres (PO2) and reducing atmospheres with gas mixtures (PCO/PCO2) [Serna, 2003].

Comparing the oxygen potential in the gaseous environment with the oxygen partial pressure in equilibrium with the evaluated oxide, it is possible to determine thermodynamically if this is stable or not under conditions worked in the laboratory.

[pic 4]

Figure 1.1. Ellingham/Richardson diagram for metal oxides. Standard free energies of formation in function of Temperature.

Source: Lay, G.Y. “High Temperature Corrosion of Engineering Alloys”. Metals Park, Ohio: ASM, 1990, pp. 47-72.

1. METAL DUSTING

The industrial environments normally connected to the metal dusting – problem contains CO, H2 and sometimes also other gaseous components, i.e. synthesis gas mixtures.

Basically there are four reactions that bring about solid carbon or coke deposition in H2-CO mixtures:

[pic 5]

[pic 6]

Reaction (2.1)  corresponds at Boudouard equilibrium and (2.2) to Steam-carbon equilibrium.

It may be concluded that during metal dusting of high alloyed steels and Ni-base alloys, three additional reactions take place which produce carbon from the gas phase according to the reactions

[pic 7]

[pic 8]

Szakálos et Al. investigated the kinetics of these reactions as a function of CO/H2 content and temperature on pure iron. The results showed that reaction (2.1) dominates at higher CO concentrations and reaction (2.2) dominates at higher H2 concentrations.

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