FCCU Reactor and Transfer Line Coking
The formation of coke deposits has been observed in virtually every unit in operation. Coke deposits are most often found in the reactor (or disengager), transfer line, and slurry circuit. They cause major problems in some units such as increased pressure drops, when a layer of coke reduces the flow through a pipe, or plugging, when chunks of coke spall off and block the flow completely. Coke deposits can be very thick; values of 3 to 4 feet have been reported(Ref. 1).
Here we discuss where coke formation appears, hypothesize how it occurs, and suggest ways to reduce or eliminate it.
Where Coking Occurs
Coke is commonly observed in the FCC reactor as a black deposit on the surface of the cyclone barrels, reactor dome, and walls. It is also quite common for coke to form stalactites from the plenum chamber or dome steam rings. Coke is often deposited on the cyclone barrels 180 degrees away from the inlet volute, i.e. out of sight. Coking within the cyclones can be potentially very troublesome since any coke spalls going down into the dipleg could restrict catalyst flow or jam the flapper valve. Either situation reduces cyclone efficiency and can increase catalyst losses from the reactor.
Coke formation also occurs at nozzles which can increase the nozzle pressure drop. It is possible for steam or instrument nozzles to be plugged completely, a serious problem in the case of level and DP instrumentation.
Coking in the transfer line between the reactor and main fractionator is also common, especially at the elbow where it enters the fractionator. Transfer line coking causes two problems pressure drop and spallingeither of which can lead to reduced throughput. The decreased area for flow coupled with surface roughness of the coke means that the transfer line pressure drop increases, resulting in more work for the wet gas compressor. If the reactor pressure is increased to compensate for the higher transfer line pressure drop, then the regenerator pressure must also increase, which in turn lowers the air blower capacity. Any coke in the transfer line which spalls off can pass through the fractionator into the circulating slurry system where it is likely to plug up exchangers, resulting in lower slurry circulation rates and reduced heat removal.
Problems Resulting from Coking
Pressure balance is obviously affected if the reactor has to be run at higher pressures to compensate for transfer line coking. On units where circulation is limited by low slide valve differentials, coke laydown may then indirectly reduce catalyst circulation. The risk of a flow reversal is also increased. In units with reactor grids, coking increases grid pressure drop, which can directly affect the catalyst circulation rate.
FCC units typically are designed for pressure relief from the reactor through the transfer line to PSV's on the main column overhead. The increased pressure drop in the transfer line from coking can have a serious effect on relief capacity.
Shutdowns and startups can aggravate problems due to coking. The thermal cycling leads to differential expansion and contraction between the coke and the wall it is adhering to. This will often cause the coke to spall in large pieces, creating the problems mentioned above.
Another hazard during shutdowns is the possibility of an internal fire when the unit is opened up to the atmosphere. Proper shutdown procedures which ensure that the internals have sufficiently cooled before air enters the reactor will eliminate this problem.
The only defense against having coke plugging problems during start up is to thoroughly clean the unit during the turnaround and remove all the coke. Since some coke deposits could be overlooked even after a thorough inspection, some refiners fit fine pump strainers on the suction line of the main column bottom pumps during startup. These will probably have to be cleaned very frequently. It might be worth considering the installation of duplex strainers or in-line "Y" strainers as opposed to the traditional "witch's hat" in the suction side flanges.
What Causes this Coke?
Coke has been observed to form where condensation of hydrocarbon vapors occurs. The reactor walls and plenum offer a colder surface where hydrocarbons can condense. Heavier boiling components in the feed may be very close to their dew point, and they will readily condense and form coke nucleation sites on even slightly cooler surfaces.
One unit we inspected during a turnaround had a thick coke layer around the dome steam ring and the steam inlet pipe, and additional coke in the form of stalactites had formed on the sparger. The steam was injected to add turbulence, thereby preventing hydrocarbons from stagnating, cooling, and ultimately forming coke. In this case, however, putting in colder, possibly wet steam perhaps had the opposite effect to that desired it introduced cold spots which allowed coke to form.
Unvaporized feed droplets readily collect to form coke precursors on any available surface. This is a more common situation than many refiners realize since equilibrium flash vaporization calculations often indicate that heavy material is not vaporized at the mixing zone of the riser. Residue processing would directionally make this problem worse.
Low residence time cracking also contributes to coke deposits since there is less time for heat to transfer to feed droplets and vaporize them. This is an observation in line with the increase in coking observed in the 1970's when short contact time riser crackers were replacing the longer residence time bed crackers previously used.
Higher boiling range, higher aromaticity feedstocks might be expected to result in worse coking rates. Laboratory studies using pure aromatic compounds indicate that polycyclic aromatics and aromatics containing heteroatoms (e.g. N, O, S) are more facile coke makers than simpler aromatics(Ref. 2). However, commercial experience shows that feed quality alone is a poor predictor of which units will experience coking problems and which will not. One interesting point about feed quality, though, is that units with feed hydrotreaters rarely report coking problems; presumably the hydrotreating step consumes or converts the coke precursors.
Certain catalyst types seem to increase coke deposit formation. Severe coking was first described in the literature when low surface area, high rare earth zeolite catalysts were introduced. Such catalysts could contribute to coking indirectly because the products they make are more likely to be coke precursors.
High rare earth zeolite tends to form aromatics from naphthenes as a result of secondary hydrogen transfer reactions. These aromatics can undergo further thermal reactions to form coke.
In addition, high zeolite, low surface areas cracking catalysts are less efficient at heavy oil cracking than the amorphous catalysts that preceded them. The older catalysts contained a matrix which was better able to precrack heavy oils and convert the coke precursors. The active matrix of some modern catalysts serves the same function. Question 12 of the Heavy Oil Processing section of the 1986 NPRA Q&A(Ref. 3) asked:
"A few years ago, we heard much about coke deposition in reactors and fractionators. Less is heard recently. Is the problem under control, or even solved?"
One of the answers given was that in response to the demand for octane, catalysts have changed from high rare earth levels with relatively inactive matrices to USY types with more matrix activity. Most units also now operate with higher reactor temperatures and increased cat/oil ratios which also help to increase the conversion of those materials which were causing the problem.
J. Mauleon(Ref. 4) points out that while USY catalysts reduce the regenerator temperature due to the heat of cracking effect, the overall effect is to increase cat/oil and the temperature at which the catalyst and oil start their progress up the riser from the mix point. These factors favor the conversion of heavy feedstock molecules and reduce the chances of their forming coke.
It therefore appears that, high rare earth zeolite, low surface area catalysts were a contributing factor in coking, and that catalysts with moderate matrix functions and USY zeolites are less likely to result in heavy coke formation.
What is the Mechanism?
McPherson in his 1984 O&GJ paper(Ref. 1) reports:
"there is evidence that the formation of coke deposits in FCC reactors and fractionators is mainly the result of thermal polymerization reactions probably involving multi-ring aromatics. A minor proportion of the reactor products is thought to combine to form high molecular weight hydrocarbons that are no longer volatile under the conditions of operation."
The formation of a liquid seems to be an important step in the coke formation process because it concentrates the reactants and provides a point of nucleation.
McPherson also sees the introduction of all-riser cracking and high hydrogen transfer catalysts as significant contributing factors. Interestingly, in 1984 he reported that the newer octane catalysts should reverse the trend; the extract above from the 1986 NPRA Q&A supports his hypothesis.
Other refinery processes besides cat crackersdelayed coking and visbreaking, for examplehave coking problems. Both operate at immensely longer residence times than FCC reactors. This factor might help explain coking in low velocity parts of an FCC unit. Lieberman(Ref. 5) describes two independent mechanisms of coke formation:
i. "thermal coke" is produced by cross-linking of aromatic rings. Needle coke, for example, is formed by the cross-linking of aromatic rings contained in the slurry oil feed to the coker.`
ii. "asphaltic coke" is formed as solutizing oils are thermally cracked away. The remaining large asphaltene and resin molecules precipitate out to form a solid structure (coke) without much change in form.
These mechanisms may well apply to coke deposit formation in an FCC unit. Any unvaporized feedstock is likely to be a heavy oil rich in asphaltenes and resins. If this material reaches a low velocity area of the reactor, the long residence time there may allow the solvent oils to slowly evaporate and form coke by precipitation.
Though the residence time is low and the yield to coke subsequently small by coker unit standards, enough coke could be formed by this mechanism to be a nuisance in an FCC unit.
How to Control Coking
The two basic principles to minimize coking are to avoid dead spots and prevent heat losses. An example of the first principle is using dome steam or purge steam to sweep out stagnant areas in the disengager system. The steam's purpose is to keep heavy, condensible hydrocarbons out of cooler regions. Steam also provides a reduced partial pressure or steam distillation effect on high boiling point hydrocarbons, helping them to vaporize at lower temperatures.
Steam is intended to sweep out any quiescent zones, not cool the system. If the presence of colder steam pipes and rings creates cold surfaces, then purge steam is not being much help.
Steam for purging should preferably be well superheated. Medium pressure steam with low velocity in small pipes with high heat losses, is likely to be very wet at the point of injection. Such poor quality steam can cause as many problems as it is supposed to solve.
An example of the second principle is using proper insulation on pipes and vessels. Cold spots are often caused by heat loss through the walls, in which case increased thermal resistance might help reduce coking. In particular check that flanges are well insulated, especially at the fractionator inlet. Exposed flanges will be a significant source of heat loss. The transfer line, being a common source of coke deposits, should be as heavily insulated as possible, provided that stress related problems have been taken into consideration.
In some cases changing catalyst type can alleviate coking problems. The catalyst types which appear to result in the least coke formation (not delta coke or catalytic coke) contain low or zero earth zeolites with moderate matrix activities.
Eliminating heavy recycle streams can lead to reduced coke formation. Since FCC clarified oil is a desirable feedstream to make needle coke in a coker, then it must also be a potential coke maker in the FCC disengager. If coking worries you, take out the oil recycle.
Design Changes to Overcome Coking
One of the most significant hardware trends in recent years has been to get improved yields via better feed atomization. Ultimately the objective is to get an oil droplet small enough that a single particle of catalyst will have sufficient energy to vaporize it. This has the double benefit of improving cracking selectivity and reducing the number of liquid droplets which can collect to form coke nucleation sites.
Total's "Mix Temperature Design" (J. L. Mauleon, et al.(Ref. 6)) specifically increases the cat/oil ratio in order to maximize the temperature at which the oil and catalyst mix. The higher the mix temperature the greater the degree of asphaltene shattering, removing the starting point for one of the mechanisms for reactor coking, as well as reducing delta coke in the process.
It is possible that the use of two-stage reactor cyclones allow more coke formation since in some designs, there is a large volume of hydrocarbons above the second stage cyclone inlet. Even if these units use purge steam it is impractical to use enough steam to significantly reduce residence time.
Some modern riser designs can help solve coking problems in the disengages. The vented riser used in the Ashland RCC unit(Ref. 2), for example, can help solve coking in the disengager.
As a modification to a T-separator, UOP(Ref. 3) installed a so-called "shot-gun'' to one unit. This consisted of a pipe which was effectively a bypass of the T-separator to blast dead zones with reactor effluents. After this modification the cyclone areas no longer suffered from coke build up.
Vessel sizing plays an important part in coking. At the initial design stage the disengager should not be made any larger than absolutely necessary since this could introduce quiescent areas. The addition of reactor overhead baffles, with steam purges behind them, is one way to reduce the effective volume of an existing vessel.
The transfer line should be as short and direct as possible, though it is probably better to allow for thermal stresses by creating U bends than attempting to use expansion joints. All sections should be free draining to prevent liquid hold up prior to the fractionator. High velocities help, but velocity has to be balanced against pressure drop and unit operating pressure when fixing the design.
How to Monitor Coke Build Up
X-rays, surface thermography and acoustic monitoring can all be used to try to determine changes which relate to internal coking.
A simple way of monitoring coke build up in the transfer line is to measure pressure drop between the reactor and main fractionator.
More sophisticated methods might include the development of a heat flux sensor which measures the amount of coking by the reduction in heat transmitted through probes in various parts of the unit.
If your unit suffers from coking there is little that can be done to radically alter the problem. One of the best ideas to improve the situation is to use improved feed injectors since better feed dispersion should reduce coking. Installation of baffles and the merits of purging steam should be considered.
Take care to remove coke during turnarounds, and during startups always have strainers in service on the suction side of the main fractionator bottoms circuit.
Coke build up in the transfer line is most easily monitored by recording pressure drop profiles.
1. McPherson, L.J.: "Causes of FCC Reactor Coke Deposits Identified"; O&GJ, September 10, 1984, pp139.
2. W. Appleby, J Gibson, and G. Good, l & EC Process Design and Development, l, 1962, pp102.
3. NPRA Question and Answer Session, 1986, (Transcripts) Heavy Oil Processing, Question 12, pp45.
4. Mauleon, J.L. & Courcelle, J.C.: "FCC Heat Balance Critical for Heavy Fuels"' O&GJ, October 21, 1985, pp64.
5. Lieberman, N.P.: "Shot Coke: its origins and prevention": O&GJ, July 8, 1985. pp45.
6. Mauleon, J.L. & Sigaud, J.B.: "Mix Temperature Control Enhances FCC Flexibility in use of Wider Range of Feeds"; O&GJ, February 23, 1987 pp52.