PTQ Q2 2023 Issue

Coking in resid cracking In the 1970s and 1980s, resid cracking was introduced as a method of disposing of an unwanted product. Coking in the feed side of the process started showing up due to non- vaporised feed. Two methods to combat the coking problem were used. First, catalyst coolers were employed to increase the catalyst/oil ratio so the coking occurred on the catalyst. The second was to vaporise as much feed as possible to minimise the oil droplets. Calculations for residual feeds indicate that as much as 25% of the feed will not be vapo- rised with conventional feed nozzles. When vaporising feed, the injectors must reduce the oil droplets to the size of the catalyst particles, minimising cok- ing seen throughout the reactor system. Coking around the feed nozzles and/or the riser walls is usually due to wet dis- persion steam, which can also damage the nozzles. Uneven feed sprays into the riser can cause coke and will have a negative impact on yields. Any cooling of reactor vapours can also cause coke. This can occur if the insulation is insuf- ficient or damaged. Start-ups can be a time when this is more likely to happen since reactor temperatures may be too low when feed is introduced. Riser design may also be a problem. If the residence time of the oil is too low, bottoms cracking may be incomplete, and the cracking that occurs in the vapour line or the bottom of the fractionator will cause coking to lay down in the affected area. Coking in the reactor cyclones has been reported on the back of the outlet tubes (see Figure 3 ), where a dead space may be present. Some refiners have put refractory on that portion of the outlet tubes to minimise coking. Unit configuration-based coking The metallurgy used might be a factor. Stainless steel could give better performance than the carbon steels typically used in this service. Uneven flow of the catalyst to the reac - tor cyclones may also produce coke in the cyclones or on the walls of the reactor vessel. Again, the unit configura - tion may be a factor contributing to the coking problem. The close-coupled reaction systems create a space above the cyclones where there is little catalyst traffic. Coke forms on the reactor cyclones because fine catalyst will deposit on the cyclones and absorb hydrocarbons, which leads to the coke build-up. Dome steam is required to prevent this coke laydown on top of the reactor cyclones. Too much dome steam can cool the reactor vapours and form coke. Wet steam must be avoided in this application since it will cause coke formation and may damage any metallurgy it contacts. The tempera- ture drop at the top of the vessel should be limited to about 10ºF. Steam rates are low, and the superficial velocity is only about 0.01 ft/sec. The distance between the steam distribu- tor and the tops of the cyclones needs to be properly sized. Each vender has recommendations for their design. Any large changes in reactor temperature can cause any coke that has formed in the process to spall off, possibly causing cyclone malfunctions or flow problems if it blocks a slide valve. Coking in the overhead vapour line is frequently due to liquid condensing on the walls. Liquid droplets caused by a feed system that poorly atomises or does not

Cyclone inlet

Coke formation on the backside of the gas o/l tubes

Cyclone inlet

Figure 3 Coke formation in reactor cyclones

The bed crackers had hydrocarbons and steam leaving the bed and going to cyclones. The top of the reactor vessel was usually cooler than the bed because cracking reactions that occur in the dilute phase are endothermic, and catalyst concentrations are low. Any polymerisation or alkylation reactions that occur can lead to condensation of the heavier products formed in the reactor. This then leads to coking on the roof of the reactor, the tops of the cyclones, and possibly the plenum chamber. Another factor was the size of the unit and how the recycle was processed. Some units had sepa- rate risers for these streams. FCC feed coking tendencies When zeolites were introduced in the 1960s, they revo- lutionised catalytic cracking. Being more than an order of magnitude more active than the amorphous catalysts they replaced meant that recycle was no longer necessary to increase conversion. Reactor beds were replaced with feed risers and a rapid catalyst/vapour separator, and the catalyst needed to be regenerated to a coke level of 0.3 wt maximum. Most units aimed for 0.10-0.20 wt% carbon on catalyst. Higher regenerator temperatures were needed, and the complete burning of CO in the regenerator bed replaced the downstream CO boilers. Feed end points for gasoils rose to 1050-1100ºF. Even higher end points occurred when the vacuum towers were pushed past design. Coking capac- ity increased, and coker gasoils were sent to the FCC. The coking tendencies of the FCC feed increase with aromatic content. Large molecules can react with the olefins and diolefins generated by the coker’s thermal cracking reactions and condense as a liquid. This can cause coke to form in the FCC reactor and all the downstream equipment leading to the main column. Coker gasoils that were sent to storage picked up oxygen which caused deposits when fed to the cracker. The reactions of the diolefins with other molecules also added to the coke formation. Hydrotreating the feed removes a lot of the coke precursors and greatly reduces coking issues.

PTQ Q2 2023