Maintaining tube skin temperature in ethylene cracker

[ivani1]

What are the best ways to effectively maintain the tube skin temperature in ethylene cracking furnace?

[sys1]

The primary control method is adjusting burner firing.
If TST increases we usually:
Reduce firing rate
Rebalance burners
Adjust fuel-to-air ratio
But there challenge like uneven flame patterns, burner fouling, maldistribution.
A single malfunctioning burner can create a localized hotspot.

[ww2i]

Decoking at the right time helps with that. Remove the coke when skin temperature approaches operating limits. Furnace is taken offline, steam-air decoking performed and heat transfer restored. After decoking, skin temperature drops significantly and run length starts again.
Poor combustion is another factor to be looked at. It can cause flame impingement, hot spots, uneven heat flux which operators normally monitor with O2 in flue gas, draft and burner flames.

[opo21]

The primary challenge in maintaining optimal coil outlet temperature is balancing maximum ethylene yield against coke formation and furnace tube life. As coke accumulates, additional firing is required to maintain COT, increasing tube metal temperatures and accelerating degradation mechanisms such as creep and carburization. Feedstock variability, burner performance, instrument reliability, throughput changes, and process upsets further complicate COT control. Successful operation requires continuous monitoring of COT, tube skin temperatures, pressure drop, and product yields while staying within equipment integrity limits.

[jeem]

Heavier feeds generally produce more coke and require higher firing duty.
Examples include; Ethane → lower coking tendency
Naphtha → higher coking tendency
Feed changes often require TST adjustments.
Just keep a check on that.

[ivani1]

I was thinking about this heavier feeds issue and adjustments. This is risky sometimes as firing duty tends to increase with an increase in tube skin temperature.