PTQ Q3 2024 Issue

reduced the regenerator catalyst inventories to 25-33% of the old designs. As shown in Table 1 , the activity of the Ecat would decline at higher temperatures, with the loss depending on the stability of the catalyst being used. Carbon on catalyst was reduced to less than 0.3 wt% to ensure the zeolite would perform, and most refiners now run less than 0.2 wt%. Elevated temperatures today would not be so high due to the lower surface areas of today’s catalysts. Coke concen- trations of 2-3 wt% are possible on fresh catalyst, and since the mix temperatures are 1,275-1,375ºF now, elevated temperatures of 150-200ºF would give peak temperatures ranging from 1,425-1,525ºF. The coke on the regenerated catalyst is on the zeolite since this is where most of the cracking occurs. Lab data (see Figure 2 ) for the cracking by zeolites would suggest that at levels around 8 wt% coke, the zeolite sites are essentially blocked. A recent study 3 indicated that at about 0.4 wt% CRC could cause lower conversions with the zeolite concentrations in the present catalysts. Reports from several FCC unit operators seem to confirm this supposition. High coke on the catalyst could cause diffusion limita- tions. Pores smaller than 50 Angstroms (Å) can cause cap- illary condensation and would increase the spent coke. If regenerator temperatures are too high, refiners will use a catalyst cooler to prevent excessive deactivation. Feed contaminates Alkali metals Feed contaminates can increase the catalyst deactivation rate. Alkali metals 4, such as sodium and potassium, can cause a severe loss of catalyst performance. Potassium poi- soning is rare but has been caused by potassium hydroxide (KOH) from the alkylation unit, which has found its way into the FCC with devastating results. Sodium is the most likely contaminant since it is found in crude oil as an emulsion of salt water in the oil. It can also come from contaminated gasoils that are shipped with salt water. Caustic added to the crude unit to neutralise acids that can form in the unit may end up in the cracker feed. It can be advantageous to desalt catalytic feed before putting it into the FCC unit if the sodium level is too high. The higher sodium content of the fresh catalyst leads to poorer stability. Most of the sodium in the catalyst is asso- ciated with the zeolite in the catalyst and causes the crystal structure to collapse at lower temperatures. This is analo- gous to using salt to lower the melt temperature of ice. The loss of zeolite activity means lower conversions, and higher bottoms yield will occur. Octanes might be lower as well since the strongest acid sites are poisoned. Nickel and vanadium Nickel and vanadium are typically found in porphyrin struc- tures that boil above 1,100ºF and can act as dehydro- genation catalysts. They also increase delta coke, which increases the regenerator temperature, thus compound- ing their deleterious effects on the operation. Nickel is the strongest dehydrogenation catalyst and has resulted in hydrogen levels of more than 300 standard cubic feet

Activity stability vs regenerator bed temperature

Approximate mat loss with temperature Regen bed temp ˚F Na-RE-Y

CREY

1,250 1,350 1,400

0 1 3

2-3 5-6 2-3

Sodium in zeolite, wt%

Table 1

(WHSV). They contained as much catalyst as the regener- ator. Modern riser reactors run with WHSVs of 50 or more, minimise the overcracking of gasoline to lighter gases, and can crack most of the reactive molecules without recycle. Coke on regenerated catalyst The early zeolite catalysts were found to be inactive when coke on regenerated catalyst (CRC) was above 0.3 wt%. Many units were designed to have 0.5 wt% CRC since there was no advantage to operating at lower values with the amorphous catalysts previously used. High-temperature It can be advantageous to desalt catalytic feed before putting it into the FCC unit if the sodium level is too high regeneration was developed to reduce the coke on regen- erated catalyst and make the unit more energy efficient. The burning rate was much higher when the regenerator temperatures were 1,250-1,375ºF vs the 1,050-1,200ºF typically used at that time. Stainless steel had to be used in all locations where equipment was exposed to higher temperatures. This

0.5

Coke content of the catalyst (wt%)

Figure 2 Activity (n-Heptane cracking)

PTQ Q3 2024