60 80 100
30000
Minutes / Cycle
5
Rx-stripper
2 2 3 18
Regen.
20
Total
5
40 30
20
20000
10
1
10
100
1000
No. of cracking-regen. cycles
Figure 5 Metals decay
10000
20
While the loss of activity is significant, the resulting cat - alysts are very operable. Since most catalysts today have 0.1 to 0.3 wt% sodium, equilibrium catalysts containing at least 0.6-0.7 wt% vanadium are operating today. Total nickel plus vanadium exceeding 10,000 ppm is common in current resid operations. Catalyst additives for passivating a metal should pref - erentially hold the contaminate. The ratio of the metal on the passivator to the metal on the catalyst is the partition factor. It should be as high as possible. Commercial results indicate that the metals are passi - vated over time in the unit. This has been correlated to the number of cycles7 that the unit makes. Figure 5 shows the data generated by Arco. Cycle times are exhibited in Figure 6 and show the cumulative cycle times for units with indi - vidual five- and 20-minute cycles. This represents the times Catalyst properties can be altered to provide resistance to sintering, including lowering the sodium in the zeolite and increasing the pore volume and pore diameters from older large FCC units and the modern high-tempera - ture designs. Iron Iron (Fe) is the latest metal that surfaced as a deactivator of cracking catalysts. Most of the iron was originally associ - ated with the clay used in the catalyst, with the rest being largely tramp iron coming from the scale off the walls of the FCC vessels. Tramp iron can cause flow problems in the secondary dip legs and increase catalyst losses. Iron was not considered to be a very active dehydrogenation catalyst in FCC catalysts. In the 1990s, Fe contamination caused serious deactivation of the catalyst and poorer circulation through the unit. Examination of the catalysts indicated nodules were formed on the surface. These nodules7 were the result of an eutectic formed
0
0
20
40
60
80
100
Days
Figure 6 Total number of cycles for FCC unit
between the iron compounds, sodium, and calcium coming in with the feed. The incoming iron is organic and shows paramagnetism at times. These eutectics are liquids at 1,300ºF that can cover the surface and block access to the interior of the particle, reducing conversion and bottoms cracking. The apparent bulk density (ABD) of the catalyst declines and the particles can lock, causing flow problems in the diplegs and standpipes. The best solution is to remove the iron in the desalter by adjusting the pH. Lowering the sodium and calcium to very low levels would also help. Catalyst properties can also be altered to provide resistance to sintering, including lowering the sodium in the zeolite and increasing the pore volume and pore diameters. The binders and other addi - tives to the catalyst might be altered to improve perfor - mance. The refiner should contact their catalyst vendors to get their opinion on the best ways to handle or remove iron. Nitrogen Nitrogen in FCC feeds is found in aromatic structures since amines are generally not present in crude oil. There are few nitrogen compounds in straight-run gasoline. Diesel contains more nitrogen, but the gasoils boiling from 650- 1,050ºF usually contain 300-1,000 ppm nitrogen. About a third of these molecules are basic. They act as temporary poisons by neutralising acid sites in the catalyst and reduc - ing conversion. Some correlations have shown that 100 ppm basic nitrogen will reduce conversion by 1 vol% when the total nitrogen is in the 700-1,500 ppm range. Gasoils from heavy crudes, such as those found in California, have high nitrogen (>0.3 wt%) and are always hydrotreated before going to the FCC unit.
80
PTQ Q3 2024
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