Date of Award

May 2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Benjamin Church

Committee Members

Steven Hardcastle, Pradeep Rohatgi, Nidal Abu-Zahra, Wilkistar Otieno

Keywords

AFA alloys, Carburization, Coking, Decoking

Abstract

Ethylene, the most-produced building block in the petrochemical industry, is produced using thermal cracking operations where reactor materials are exposed to high temperature aggressive environments that limit their life expectancy. These atmospheres degrade the reactor's material due to exposure to high temperatures, to carbon, to sulfur, and to other compounds, affecting negatively its process operations and therefore, its economics.

One of the most undesirable phenomena in ethylene thermal cracking operations is coking, where a carbon layer is deposited on the walls of the furnace tubes causing a reduction in the process efficiency as well as degradation of the alloy. This leads to an inevitable shutdown of the furnace tubes, or decoking, where steam is introduced to remove the carbon layer, causing higher operational costs and less energy efficiency of the plant. As a result, furnace tubes deteriorates with the successive coking-decoking cycles due to corrosion and carburization reducing the lifetime of the reactor material, and negatively impacting the process economics.

Currently, thermal cracking reactors are made of austenitic Fe-Ni-Cr heat resistance steels, or chromia-forming alloys, which form a protective chromium oxide layer during exposure. These alloys rely on chromia scales for protection from high temperature oxidation but their performance is limited in many industrial environments. More recently, research into materials which produce protective layers of aluminum oxide, or alumina-forming alloys, have been explored as a way to further slow the rates of high temperature oxidation and coke build-up. While studies on chromia-forming alloys used for ethylene production are widely available, there is a lack of published scientific work understanding the performance of alumina-forming alloys in these petrochemical applications.

In this work, the coking resistance of several alumina-forming (3.2 to 4 wt% Al) alloys relative to a traditional chromia-forming (HP) alloy is described. An analysis of oxidation rates, coking kinetics, microstructural impacts of long-term exposure to coking conditions, and coking-decoking cyclic atmospheres are considered in this study. Through microstructural analysis, mechanical testing and kinetic studies, the materials were analyzed to characterize oxide layer formation, carbon build-up, carburization, and changes to the base metal microstructure.

Overall, the alumina-forming alloys showed superior performance in coking-related environments relative to the traditional HP alloy. Results show that the alumina-forming alloys gained approximately 80-95% less mass than that of the chromia-forming alloy after exposure to static coking and cyclic coking-decoking conditions. In addition, the AFA alloys had not major microstructural changes. In contrast, higher carbon build-up and severe carburization was found in the HP alloy. Furthermore, coking kinetic parameters were evaluated at 900, 950, and 1000 °C, in a ethylene-hydrogen atmosphere, and an estimation of the apparent carbon diffusion coefficient in the matrix of both AFA and HP alloys were obtained.

The information developed in this study will better support the use of AFA materials in petrochemical applications, such as ethylene thermal cracking operations. Additionally, coking kinetics and diffusion parameters of AFA alloys have now been measured, allowing for these parameters to be used as the metric for coke formation and relative indication of performance. Therefore, with a better understanding of the coking resistance of alumina-forming alloys, a higher productivity and a reduction in the operational costs can be achieved when implemented in thermal cracking operations.

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