If the cracker is operating at a catalyst circulation limit, a revamp should be considered. Packings have been found to improve the mixing of the catalyst and steam and run with a constant catalyst velocity based on the superficial velocity of the vessel. An open area of about 96% allows this to happen. High flux rates are well below the rise velocity of a one-inch bubble, and the strip- per will perform hydrodynamically. Residence times of the catalyst will be shorter at the higher flux rates, so a higher bed level with heavier feeds may be necessary. Other vari- ables that can be manipulated are the reactor temperature, the pore volume, and the pore structure of the catalyst. More dispersion steam might also be tried. Stripper problems The operation of the stripper may become unsatisfactory and require some troubleshooting. The amount of steam going to the stripper should be checked. If the regenera- tor temperature has increased, it is possible the steam rate has diminished. This would allow hydrocarbons into the regenerator, which would burn hotter than the coke. Some refiners have tried to control the regenerator temperature by adjusting the steam rate. When the delta coke was too low for the desired regen- erator operation, an easy fix was to cut back the stripping steam. More than one refiner has tried this and gone too far, with the result of a very high regenerator temperature (>1,600ºF). The resulting ecat was a 40/40, a total surface area of 40, and an activity of 40. Hydrotreating the feed was a frequent cause of this situation. Adding a cat feed fired heater may be the best solution to rectify the problem. There are better solutions available to correct this prob- lem. The burning of reaction products is expensive and is just too difficult to control. Raising the feed temperature will work, and the use of CO combustion promoters to let the CO burn in the regenerator bed rather the dilute phase has been found to be effective. Increasing the bed depth has helped to keep the CO conversion to CO₂ in the catalyst bed. Higher catalyst activity or special high delta coke addi- tives will raise the bed temperature and are frequently used when bed temperatures need to be increased. Problems with the steam distributor can cause maldistri- bution of the steam and spent catalyst. A power outage or the loss of steam could lead to defluidisation of the stripper bed and the plugging of some of the nozzles or the distrib- utor itself. When the steam and/or power is lost, the dis- tributor can fill with catalyst and may not clear itself when the equipment is restored to working order. If the steam condenses, the wet catalyst may clump and plug orifices. It helps to have drain holes in the distributor and some down- ward-directed nozzles to refluidise the stripper bed below the distributor. All steam should be superheated, and water drains should be regularly used. In extreme cases, the steam distributor breaks off, and all the steam goes up one side of the stripper. With disc and dough- nut designs, a fast turnaround is required to fix the unit. With packing, the steam redistributes as it goes up the stripper. By the time the fourth stage is encountered, the two phases are mixed well again. The unit is operable, but the number of
Stripper improvement
8% H in coke
6% H in coke
ROT, ºF
980
980
Regen bed, ºF
1357 550 5.87 0.84 Wt% 4.46
1339 558 6.13 0.83 Wt% 4.21
Feed preheat, ºF
C/O
Delta coke
Vol%
Vol%
Dry gas (+H2 S)
Total LPG
18.10
29.12
18.29
29.47
Gasoline, TBP cutpoint 680ºF LCO, TBP cutpoint 430ºF
51.19
61.51
51.76
62.14
16.47
15.95
16.06
15.52
DO
4.85 4.93
4.03
4.57 5.11
3.79
Coke
Conversion
78.68
80.02
79.37
80.69
Table 2
operating stages may be too low to get the needed overall efficiency. This is indicated by the data in Table 1. Verifying damage If there is suspected damage to the internals of the stripper, tracer studies can be carried out to check both the catalyst and steam flows. Distribution of these components across the bed both horizontally and vertically throughout the stripper bed will provide key information. Gamma scans can be used to find bed heights and standpipe hydrodynamics. The level in the stripper may be a factor when the unit is running. If the level is too low, the residence time may be insufficient to remove all the hydrocarbons. This could also result when a heavier feed is being processed or the unit throughput is well above design. If the level is too high, the catalyst flow may become erratic. If the cyclone diplegs are buried in the bed, the aeration around the diplegs may be impacted, such that catalyst separation is compromised. The cyclones would have higher dipleg levels and could even flood. High catalyst losses to the main fractionator might also be seen. Deeper fluid beds promote more channelling of the gas. Each stripper design has to be reviewed to locate potential operating limits.
Bubble rise velocity
V B = 0.711 x g c D B
Bubble size
Velocity (Ft/sec)
(inches)
1 2 4 6
1.16 1.65 2.33 2.85
Table 3
75
PTQ Q1 2025
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