Coking (process fouling)

Coking in refining and petrochemicals is the formation of hard carbonaceous deposits on hot process surfaces - typically inside ethylene-cracker furnace tubes, delayed-coker drums, and the radiant tubes of fired heaters. Coke forms by thermal cracking of hydrocarbons in stagnant or low-velocity zones, accumulating until a planned decoking outage removes it.

Where it dominates

• Ethylene-cracker furnace radiant tubes
• Visbreaker furnaces
• Delayed-coker process drums
• Some refinery heater tubes
• FCC catalyst (different mechanism - burned off in the regenerator)

Cleaning

Coke is hard, bonded, and refractory - far beyond what sonic horns can address. Standard cleaning is by decoking: a campaign in which the heater is run with a steam-air mixture at elevated temperature, oxidising the deposit out of the tubes. Manual mechanical pigging is sometimes used on selected sections.

Acoustic cleaning is not a primary tool against coking, but downstream particulate-handling equipment (decoking-effluent dust collection, SRU adjacency) can benefit from sonic-horn coverage.

Mechanism

Coking is driven by thermal cracking and polymerisation of hydrocarbons. Heavy molecules break down at high temperature, form reactive fragments, and eventually leave a carbon-rich solid on metal, refractory or catalyst surfaces. Low velocity, stagnant zones, hot spots and long residence time make the deposit worse.

The mechanism is different from ordinary ash fouling. Coke is chemically bonded, hard and often grows from the process fluid side of tubes or vessels. It changes heat transfer, narrows flow area and creates higher tube-metal temperature, which then accelerates further coking.

Where it matters

Ethylene crackers, delayed cokers, visbreakers, refinery heaters and some reformer services manage coking as a normal operating limit. In fired heaters, coke inside tubes reduces heat transfer to the fluid, forcing the tube wall hotter for the same duty. In cracker furnaces, excessive coke raises pressure drop and lowers selectivity before a decoke is scheduled.

Catalytic units use the same word differently. FCC catalyst coke is expected and is burned off continuously in the regenerator. Heater-tube coke is not expected continuously; it is removed by planned steam-air decoking, mechanical pigging or specialised cleaning.

Relationship to acoustic cleaning

Sonic horns cannot remove internal tube coke because the deposit is inside pressure parts and chemically bonded. They may still have a role in the external flue-gas path of fired heaters, boilers or waste-heat sections where ash or dust accumulates on accessible gas-side surfaces. In coking service, it is important to separate process-side coke control from gas-side fouling control before selecting cleaning technology.

Field checks

Coking is diagnosed by temperature history, residence time, hydrocarbon composition and deposit appearance. In furnaces and process heaters, high tube-metal temperature or poor flow can crack hydrocarbons into hard carbonaceous layers. In delayed coking, the term is deliberate and controlled. In ducts or gas-cleaning equipment, however, carbon-rich sticky deposits can trap dust and create a fouling layer that is difficult to remove online.

Maintenance checks depend on where the coking occurs. Heater tubes may need pressure drop trending, tube-skin thermography and decoking cycles. Tar or pitch-rich gas paths need temperature control to avoid condensation and ignition risk. Acoustic cleaning has limited value against dense, baked carbon stuck to hot metal, but it can help where coking creates a brittle dust-rich layer or where dry coke fines bridge in hoppers and ducts. The honest boundary is adhesion: if the deposit is chemically bonded or tar-wet, mechanical or thermal cleaning is usually required.

Related terms

• Fluid catalytic cracking (FCC)
• Reformer furnace
• Fouling