Successful Automation Strategies to Prevent Brittle Fracture in a Bimetallic Demethanizer

Older demethanizer towers are commonly constructed of two different types of metals, sufficient to handle normal operating conditions: Stainless steel in the colder top section and carbon steel in the warmer bottom section. Risk of cold temperature embrittlement and brittle fracture can be initiated if the cold liquid from the top section reaches the bottom section or if there is a sudden loss of pressure in the column. The process is more likely to reach those conditions during upsets or transitions between operating states, where automated process controls can be used to help provide safeguards and reestablish stability. In accordance with the industry practice at the time of the original Port Arthur facility Light Olefin Unit design in 1978, the embrittlement risk was mitigated by inclusion of a “warm up” line that would route 60 degree Fahrenheit vapor from cracked gas dryers to the demethanizer tower to heat it when needed. In 2006, a site incident related to corrosion under insulation on that process line instigated the development of alternative strategies. The automated control was built up over time from the simple warming vapor injection to a complex multi-prong approach that successfully avoided cold temperature embrittlement operating regimes and incidents. In 2015, the bimetallic tower was replaced by one entirely constructed of stainless steel as the final iteration of risk reduction efforts.