Catalysis 2024 Issue

Reactors The feed and reactor system can have a real impact on delta coke. Some of these may be adjusted while operating the unit, but others would need a redesign and revamp of the cracker. A straight vertical feed riser will give a lower delta coke than one with bends in it. A paper by Voohries showed that the carbon on catalyst produced in the reactor was a direct function of the residence time of the catalyst in the reactor. All the reactors used in catalytic cracking during the first 20 years (1942-1962) were dense bed, stirred tank reactors. Data correlated with the catalyst residence time to the 0.5 power. After zeolite catalysts were introduced, refiners converted their units to riser reactors to shorten the residence time, and the correlations showed that the exponent dropped to 0.2. Riser vs catalyst residence time The catalyst residence time is a function of the slip factor: the difference between the catalyst and vapour residence times in the reactor. The catalyst flows slower than the vapour due to gravity and the friction created as it moves towards and up the riser wall. Bends in the line tend to throw the cata- lyst to the wall. The core-annular flow that occurs in risers also contributes to this phenomenon. The vapour flow is not flat but tends to be parabolic, with the highest velocity in the middle and the lowest at the wall. This produces the wall effects seen in risers. When the catalyst residence time increases, the pressure drop goes up, and the delta coke will be higher. The preferred design would be a straight vertical riser or a downer where the catalyst falls directly down a vertical tube. The latter might have a smaller catalyst accel- eration zone and even less slip. Riser vs dispersion steam The number of feed nozzles is important. Experience has shown that more nozzles provide better coverage of the riser cross-sectional area. This minimises thermal cracking and will reduce both dry gas and delta coke. Increased dispersion steam might reduce the delta coke by lowering the residence time of the catalyst in the reactor. This may actually reduce conversion if the residence time is already low. Bottoms cracking may also suffer with heavier feeds that require more time to crack the larger molecules. Any increase in dispersion steam should be made after look- ing at the tip design to see what impact this might have on catalyst attrition. Lift gas can also be used to produce the same effects as the dispersion steam. This gas is added at the base of the riser and may lower the dry gas and delta coke. Depending on the layout of the catalyst circuit and the metals on the cat- alyst, the preferred use of steam for this service might have to be replaced with dry gas to minimise catalyst deactivation. Recycling dry gas will add to the wet gas compressor load. In some cases, reactor internals might be considered. These are added to the interior of the riser to reduce the residence time and, hence, the delta coke. These can have an impact on the unit pressure balance since the reactor pressure drop will change. If it increases, the regenerated catalyst slide valve delta P will decline unless the delta P between the regen and

thermal cracking would occur, and the dry gas would increase. In the past, some refiners injected water into the regen - erator bed to reduce the temperature. Catalyst deactivation prohibits this practice. Adding water to the reactor works but makes a lot of coke due to the increased heat demands. For low regenerator temperatures, more preheat will raise the regenerator temperature. This can be done with heat exchange up to about 550ºF. A fired heater is required for higher temperatures with the limit set by the coking rate in the fire heater. Most units can run up to about 720ºF, though those with lower end point feeds can operate at 750ºF. Catalytic cracking units with feed pretreaters should consider a heater since the coke demand to run the process is only about 3-4 wt% of the feed. Oxygen can be added to the air to increase the coke burn- ing capacity. It will raise the regenerator temperature by about 10ºF for every per cent increase in the oxygen content of the air. The lack of nitrogen reduces the heat loss out with the flue gas and makes the process more energy efficient. Torch oil can be used continually, but if used improperly, it will deactivate the catalyst. Spray nozzles should be retractable, flow rates monitored, dispersion steam used, and torch oil should always be added to the dense bed well above the air distributor but never into the dilute phase. Safety protocols are needed to ensure any unit upset will stop the torch oil flow. FCC catalyst All cracking catalysts produce coke as a consequence of the catalytic reactions. Every catalyst will make more coke as their activity is increased. A high delta coke catalyst will raise the regenerator temperature, while a lower delta coke catalyst will reduce it. This delta coke difference can be quite large, with changes of 120ºF being observed with a change of catalyst going to a commercial resid cracking unit. The active sites on the catalyst are closer together on the higher activity catalyst. Coking reactions tend to be bi-molecular, so spacing the sites farther apart will lower the coking tendency. Since feeds become harder to crack at higher conversions, a com- parison of catalysts should be made at similar conversions so both candidates see the same feed. The equilibrium cat - alyst activity is a function of its initial activity and its deac- tivation rate. Deactivation rates go up with regenerator temperature, the steam partial pressure in the regenerator, and the age of the catalyst in the circulating inventory. The level of contaminates that deposit on the catalyst can also have a big effect. Metals such as Ni, V, and Fe can cause problems, and alkali metals such as Na, K, and Ca can compound the damage. Catalyst suppliers should be consulted when these compounds are present in the feed. They have a lot of experience with their products in many different operating conditions and can make helpful sug- gestions to counter the extraneous feed components. Equipment used The layout of the equipment can influence the delta coke that is made during operations.


Catalysis 2024

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