ABSTRACT
Coking is the process of carbon deposition from a gas phase that is encountered in many reforming, cracking and other high temperature processes. Coking in certain petrochemical processes can lead to carbon build up causing reduced process efficiency, corrosive attack and degradation of the alloy. Components used in these processes are fabricated from HP alloys that form a chromia-based oxide layer or more recent alloys that form an alumina-based oxide layer to help protect against coking.
An experimental high temperature coking atmosphere was constructed and used to evaluate the effects of temperature, time and metal surface roughness on the carbon deposition of an alumina forming alloy. Coking conditions were simulated with multiple atmospheres including CO-H2 mixtures at moderate temperatures and ethane at higher temperatures. Carbon deposition was tracked using specific mass change of the samples as a function of exposure times and conditions. Results obtained with the alumina forming alloy were compared to a baseline HP alloy. The materials were analyzed using XRD, SEM, and optical microscopy to characterize the oxide layer formation, carbon deposition layers and carbon attack, and changes to base metal microstructure. Raman spectroscopy was used to characterize the carbon deposits. The overall resistance of the alumina-forming alloys relative to the traditional chromia forming composition will be described.
INTRODUCTION
The ethylene production process and other petrochemical processes can be improved by developing materials that resist coking, retain long-term oxidation and corrosion resistance, and yet remain economically favorable for implementation. Typical alloys used in these applications are austenitic Fe-Ni-Cr heat resistance steels which form a protective chromium oxide (chromia) layer during exposure. The chromia layer acts as a diffusion barrier that restricts the transport of gas-phase constituents (oxygen, carbon) and alloy constituents (Fe, Ni, Cr, and others) so that the inevitable reactions between the gas and solid are slowed.1,2 These heat resistant stainless steels rely on chromia scales for protection from high temperature oxidation but their performance is limited in many industrial environments. Depletion of chromium in the alloy due to carburization can degrade the alloy's ability to regenerate a protective oxide scale thus resulting in faster coke build-up and further carburization.