Date of Award

December 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Benjamin Church

Committee Members

Pradeep Rohatgi, Steven Hardcastle, Xiaoli Ma, Hamid Seifoddini

Keywords

Alumina, Chromia, High Temperature, Kinetics, Oxidation, Petrochemical

Abstract

Petrochemical industries produce 14.33 million barrels of petroleum products a day and by 2023, are expected to produce over a trillion dollars in sales annually. Petroleum is the number one used fuel source and is the raw material used to produce a wide range of petrochemical products including ethylene which is the raw chemical precursors that is crucial to the polymer market. Ethylene is created by the process of cracking ethane and other hydrocarbons in steel reactors at high temperature and potentially oxidizing conditions. The cracking process produces a range of byproducts including a detrimental solid carbon, called coke, which forms on the inner walls of the ethylene cracking reactor. The coke coats the reactor and thus reduces the efficiency of the process. The coke can also attack the steel reactor material through various degradation mechanisms such as carburization, metal dusting, or related carbon-based attack. Petrochemical processors periodically stop the cracking process to “de-coke” the reactor by flowing a mixture of high-temperature (~850 C) steam and air to remove the coke deposits. This cleaning process costs the petrochemical industries over 1.3 billion dollars a year due to downtime. Additionally, the repeated cycling between carbon-rich atmospheres and oxidizing atmospheres can further damage the steel reactor tubes and reduce their lifetimes which, over the long term, create additional costs for the producers. Ethylene production reactors have traditionally used cast heat resistant austenitic stainless-steel alloys such as HP. These traditional materials form chromium oxide scales during exposure to oxidizing conditions and are classified as chromia-forming austenitics (CFAs). Chromia has been shown to have poor chemical resistance to steam conditions in high temperatures and is vulnerable to carbon- and sulphur-containing environments (sulphur being a potential contaminant in petrochemical feedstocks). One promising option to combat the problems is using a modified class of heat-resistant austenitic steel that is designed to form aluminum oxide scales upon exposure to oxidizing conditions. These alumina forming austenitic (AFA) alloys are intended to offer better chemical resistance to the steam and carbon-rich atmospheres found in petrochemical processing applications. As these AFA alloy systems are relatively new, detailed studies comparing the performance of CFA and AFA materials are needed to support the wider adoption of the new AFA materials in petrochemical processing systems. AFA and CFA materials were obtained from an industrial scale production and not fabricated in the lab. Oxidation performance of AFA materials was examined in the temperature range of 700 ~ 1100 °C with a focus on oxidation kinetics, observation of microstructural degradation and the analysis of oxide scale formation. Results are compared to the traditional CFA material, HP. Parabolic rate constants are determined by high temperature studies at 800°C, 850°C, and 900°C in steam and air-steam environments and are used to calculate kinetic activation energies. Furthermore, additional data such as diffusion profiles of specific elements can be determined. This will reveal conclusive data on the performance of AFAs and contribute to the first thorough study of its oxidation kinetics in steam atmospheres. Initial results show AFA samples have smaller mass gains compared to CFA which indicates superior oxidation performance. Furthermore, AFA samples containing roughly 3~4 wt% Al were shown to form uniform alumina scales when oxidized. The microstructural integrity of AFA materials near the surface had less corrosion damage compared to CFA alloys. Additionally, activation energy of AFA in steam is higher than CFA which indicates the AFAs are more stable over longer periods. Longer oxidation tests and variation of air to steam environments will be performed to provide data that simulates petrochemical oxidation conditions and comprehensive comparison to CFA. These oxidation kinetics results will provide a foundation that will be used to predict the material’s lifetime usage. Furthermore, the comparison of the AFA to CFA is crucial in convincing a wider adoption of the application of AFA alloys in industrial use. The research study will help bring multiple stake holders in the investment of petrochemical/ethylene production industries, metal foundries, and academic research.

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