Base
85
NI
80
-10
75
V
70
-20
65
60
-30
50
0
0.2
0.4
0.6
0.8
1.0
Nickel or vanadium on catalyst (wt%)
+0.3
Vanadium + sodium (ppm)
NI
Figure 4 Benchmarking data of Ecat activity vs vanadium and sodium
+0.2
V
level was so low, it was decided it should be stopped all together. Hydrogen production only went up about 20 scfb, which could easily be handled by the wet gas com- pressor. There are safety considerations with antimony usage, and these were mitigated with its removal from the unit. Antimony additives can come with several different carriers. These carriers should not contain molecules like sodium, which could cause more problems. Nickel lands on alumina and stays relatively fixed to the catalyst surface. Vanadium is much more mobile and can assume a number of oxidation states. V 2 O 5 can form vanadic acid, migrate to the zeolite, and cause the crystal to collapse. Some vanadium compounds are a liquid at regenerator temperatures and can migrate throughout the catalyst particle and move to other particles. Early catalyst testing in the lab showed that 5,000 ppm vanadium would cause serious catalyst deactivation. The unpassivated effects of nickel and vanadium on catalyst activity and hydrogen production are shown in Figure 3 . The industry searched for an effective passivator, and it was discovered in the laboratory that magnesium and calcium neutralised vanadium. Commercial trials, however, did not show any passivation. The magnesium and cal- cium vanadates were not as stable as the corresponding magnesium or calcium sulphates. Of the two, magnesium showed some passivation effects. Rare earth elements will form stable compounds with vanadium and are offered as vanadium traps by several vendors. The impact of vana- dium plus sodium on FCC catalyst⁶ is shown in Figure 4 . These carriers should not contain molecules like sodium, which could cause more problems Antimony additives can come with several different carriers.
+0.1
Base
0.8
1.0
0
0.2
0.4
0.6
Nickel or vanadium on catalyst (wt%)
Figure 3 Nickel and vanadium effects on FCC catalysts
per barrel (scfb) when it was not passivated. Copper is as active as nickel and has no known passivator, but the level seldom exceeds 100 ppm. Normal hydrogen content from the FCC unit usually ranges from 20-45 scfb. When levels exceed 60 scfb, pas- sivation might be considered and should be used when hydrogen reaches 100 scfb. Boron has also been cited as an effective nickel passivator that reacts with nickel and prevents catalyst dehydrogenation. Both nickel and vana- dium can be at least partially passivated with catalyst for- mulations or separate additives. Antimony is very effective with nickel. Phillips Petroleum discovered this passivator, which was introduced as a liquid additive and pumped into the feed prior to the feed injectors. Antimony could also be impregnated on the catalyst. Experience showed that the liquid additive was more effec- tive and allowed the unit to respond better to varying feed nickel levels. The maximum amount of antimony added was equal to one half the nickel level. Surplus antimony would go to the bottoms stream from the main fractionator and has caused erratic operation of the bottoms circuit. The anti- mony formed a compound with the nickel that prevented the oxidation state of the nickel from becoming an active dehydrogenation catalyst. Each catalyst used will show a unique antimony-to-nickel ratio on the equilibrium catalyst. In the 1980s, a large pore alumina 5 added to the cata- lyst showed the ability to passivate nickel. Its original intent was to lower delta coke, which it did in all FCC operations. When the concentration of the alumina increased, the anti- mony level on the catalyst declined even though the nickel level was steady at about 3,000 ppm. When the antimony
79
PTQ Q3 2024
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