(41c) Quantifying the Risk of NOx Accumulation in the Ethylene Cold Box from the Use of Polymerization Inhibitors

In the 2018 EPC meeting, Dow Chemical presented a paper, “Cold Box Fouling in a Cracker”, where Dow reported that “the main contributing root cause for the cold box plugging was identified as the co-cracking of a C4-recycle stream from the Butadiene plants containing NO containing inhibitor residuals leading to NOx formation in the furnaces.”

Pilot plant tests were conducted to quantify the relationship between dosage of inhibitors in the furnace feed and the concentration of NO in the furnace outlet under cracking and hot steam standby conditions. The tests were conducted in a pilot plant reactor designed to be operated under the same process conditions as commercial cracking furnaces (cracking severity, residence time, pressure drop, and coil outlet pressure.) Results from this pilot cracker can be transferred to commercial plant operation.

In the baseline condition without inhibitor, the concentration of NO in the furnace outlet was below detection limits. When 50 ppm of two commonly used commercial inhibitors containing N-O bonds were added, NO was detected in the furnace outlet under both cracking and hot steam standby condition. Material balance calculations indicate accuracy of the results within 5% based on the chemistry of the inhibitor added.

New inhibitor technology was also tested for polymer control in Butadiene system as well as in pilot furnace. In tests of the new technology, NO was not detected in the furnace outlet. Polymerization inhibition studies conducted in lab also suggested satisfactory performance of this new additive compared to N – O based commercial inhibitors. The AICHE Task Group for NOx Safety has advised the industry that any ethylene plant cold box is potentially vulnerable to accumulation of NOx compounds, and plans to mitigate the potential hazards of this accumulation need to be in place. Substitution of the new inhibitor that does not contribute to NO formation in the furnace offers the industry a new option for reducing the risk of NOx accumulation.