US EPA

8336300 

Meetings & Symposia 

Analysis of thermal stress and optimization of alcohol wash procedures for removal of NOx gums/salts in BAHX (cold boxes) 

Mcneilly, A; Decker, D; Peterson, D 

2013 

465-476 

English 

NOx gums/salts are potentially unstable compounds that ethylene production facilities should 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. Experimental analysis has shown that NOx gums/salts can decompose with an energy density similar to TNT. Operators can conduct precautionary procedures to remove NOx gums/salts from production equipment and minimize the risk of explosion. This is often achieved by flushing internal passages of equipment where NOx gums/salts are suspected of accumulating with alcohol. This serves a dual purpose to dissolve the compounds and provide a heat-sink before the equipment temperature rises above the temperature where NOx compounds can become unstable. Brazed aluminum plate-fin heat exchangers (BAHX, commonly installed in '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 alcohol 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 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 hazards and does not damage a BAHX.