The formation and buildup of carbonaceous deposits (coke) on metal surfaces in refinery or petrochemical plants can be a major issue impacting the performance of the unit. For example, in an ethylene steam cracker, the coke deposition and buildup will increase the required process temperature, which will result in metallurgical damage to the reactor coil with undesirable side reaction products.
Premature rupture is also possible and can be caused by carbon ingress (carbon atoms diffusing into the metal). This results in a brittle carburized layer at room temperature. Any cracking through that carburized layer can locally reduce the mechanical integrity of the metallurgy. Additionally, there can be a pressure drop increase because of the reduction of the inner diameter of the coil due to the coking. For that reason, operators must periodically perform a decoking operation that reduces production and can be deleterious to the metallurgy. Additives to prevent coke formation have the potential of causing corrosion issues. Below, we’ll discuss the mechanisms behind the formation of a carbonaceous deposit and how to minimize its formation.
Thermodynamic carbon activity will determine whether there will be carburization, coke deposition, or metal dusting. Carbon activity of less than 1 results in just the metal being carburized, while carbon activity of around 1 will convert the carbon source to graphite on the metal surface and lead to coke formation. Much higher carbon activity will produce the catastrophic phenomenon of metal dusting. The rate of formation of coke on a metal surface is time dependent. Initially, it’s at a faster rate due to catalytic wall effects and ultimately reaches an asymptotic rate of coke deposition.
There are several factors impacting coke formation:
- Feedstocks
- Temperature
- Reaction conversion
- Reactor material of construction
- Residence time of the carbon source
- Partial pressure of the carbon source
The greater the concentration of dienes in the feed increases the carbon activity and the potential for carbonaceous deposit formation. Acetylene has a higher propensity to produce pyrolytic coke deposits than does ethane. Long residence times allow secondary reactions, which can produce coke precursors. In some feeds (depending on temperature), parts per million (ppm) levels of propadiene can cause severe coke buildups. Often, it is the reaction products that are the critical species for the development of undesirable carbonaceous deposits.
There are three basic coke formation mechanisms on a metallurgical surface.
1. Catalytic Coking
The catalytic coking mechanism involves the hydrocarbon being chemisorbed onto the metal surface via a metal dehydrogenation reaction that forms graphite. The carbon atoms can then dissolve and diffuse into the metal. With further carbon accumulation, there is a pressure increase at dislocations and grain boundaries causing a small metal particle to be lifted from the surface. Hydrocarbons can then react further at the base, thus lifting the particle and producing a graphitic stem. The result is referred to as filamentous or metal-catalyzed coke. The precipitation of carbon can produce structural deficiencies in the filament, which can create reactive carbon centers along the skin of the filament. Lateral growth of the filament is a result of hydrocarbon radicals and molecules from the gas phase being incorporated at these reactive sites. Thus, a layer of loosely interwoven filaments can transform into a hard carbonaceous scale.
The metallurgical surface is very critical to the formation of metal catalyzed coke. Fe and Ni alloys are excellent metals for the dehydrogenation of hydrocarbons. On the other hand, oxides of Cr, Al, and Si cannot dehydrogenate hydrocarbons. However, under highly reducing conditions, alloys containing these alloying elements may no longer be able to prevent metal catalyzed coke formation.
2. Pyrolytic Coking
The second mechanism is by free radical formation or pyrolytic coke. Hydrogen, methyl, and ethyl free radicals are the most active species, reacting via addition to form an aromatic-type ring structure. This tar-like substance can then stick non-preferentially to the metal surface. Continued addition of free radicals increases the thickness of the carbonaceous deposit. This type of coke mechanism may also be associated with the initiation of the coke deposition for metal catalyzed coke formation. Again, trace amounts of acetylene or dienes can enhance this reaction mechanism. This type of coke tends to have very low hydrogen content.
3. Condensation Coking
The third mechanism or condensation mechanism is based on the formation of polynuclear aromatics in the gas phase formed from free radical reactions as noted above. The aromatics are produced by trimerization and other reactions involving acetylene. Starting with simple aromatics, condensation and dehydrogenation reactions produce tar droplets or soot particles depending on the reactor conditions. These tar droplets then stick to the metal surface of the reactor. Additional condensation reactions grow the coke further. Feed stocks such as heavy naphtha and vacuum gas oil are prone to this type of coking mechanism. Although the exact temperature for this mechanism is not well defined and is a function of the process conditions and hydrocarbons present, it’s considered to become prevalent around 700°C. However, early stage of coke formation at 600°C has been observed via the Diels–Alder reaction.
Of the three mechanisms, the metal catalyzed or filamentous carbon grows the fastest and is often the reason for an unexpected shutdown. For thermal cracking reactors such as steam crackers, the primary mechanism is by free radical formation. Absolute coke formation rates tend to be acetylene > olefin > aromatics > paraffins.
For condensation-type coke formation, heavier feedstocks are more susceptible. Tar-like droplet fouling on downstream colder sections can also be an issue.
Identification of the Mechanism
Scanning electron microscopy can be used to differentiate the filamentous structure with metal hats for the metal catalyzed coke from the “popcorn-like” surface morphology of the pyrolytic mechanisms. Pyrolytic coke tends to be amorphous, while metal catalyzed coke is a mix of amorphous and ordered structures as determined by Raman spectroscopy. The location of the filamentous coke may not be where it initiated.
Although not practical for commercial applications, a highly polished surface can reduce the potential of tar-like substances initially sticking to the surface. A more useful approach is to minimize high residence time locations.
The use of coke inhibitors can minimize filamentous coke formation; these include sulfur (S) compounds added to the feed at very low levels that decompose to H2S. A typical additive is dimethyl disulfide or dimethyl sulfide. The S reacts with the metallurgy to produce a sulfide scale that reduces metal catalyzed coke formation and carburization of the base metallurgy. If used with austenitic stainless steel metallurgy, neutralization is required with every shutdown to prevent polythionic acid stress corrosion cracking. To some extent, S can also reduce the gas phase coking rate by terminating free radical reactions. Organophosphorus compounds have also been found to be reasonably effective.
Oxides such as Cr2O3, Al2O3, and SiO2 can minimize filamentous coke formation. However, these tend to reduce under some process conditions. An impervious, non-porous barrier coating can be applied to the metal surface and can be effective but has limited in-field application, such as field repairs.
Conclusion on Carbonaceous Deposit Fouling
Coke formation and deposition can have a profound effect on the performance of a process unit. This includes posing a serious threat to operational efficiency, equipment integrity, and overall plant safety. Left unchecked, carbonaceous deposits can cause increased pressure drops, metallurgical damage, and even premature failure of reactor components.
Of course, complete prevention of coke deposition may not be feasible in all industrial scenarios. However, understanding and identifying the mechanism of the deposition can help determine the potential remediation that is required. This allows operators to slow its progression, schedule more effective maintenance, and ultimately extend reactor life.