NOx gums/salts are potentially unstable compounds that ethylene production facilities must include in their safety program. These compounds can form at cryogenic temperatures and high pressures, and can form deposits in equipment. NOx gums/salts are unstable at elevated temperatures, with certain species being unstable at ambient temperatures and colder, and can spontaneously decompose with an energy density similar to TNT. Operators can conduct regular maintenance to ensure that NOx gums/salts are neutralized and removed from production equipment to minimize the risk of explosion. This is often achieved by flushing internal passages of equipment where NOxgums/salts are suspected of accumulating with alcohol, which dissolves the compounds, before the equipment temperature rises above the temperature where NOx becomes unstable.
Brazed aluminum plate-fin heat exchangers (BAHX, commonly known as 'cold boxes') are susceptible to NOx gum/salt accumulation due to equipment operating at cold temperatures and high pressures that favor their formation. Flushing cryogenic temperature equipment with ambient temperature methanol raises concern about potentially thermally shocking and damaging the BAHX. To address this issue, Chart Energy and Chemicals (a manufacturer of cold boxes) and a close industry partner investigated an in-field BAHX undergoing an alcohol wash event. This paper details methods used including a dynamic 3-D analysis for flow, thermal and structural simulation. Tips are provided on practices to avoid. Finally, a set of general guidelines applicable to general alcohol wash procedures is offered that mitigates NOx gums/salts instabilities and does not damage a BAHX.
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