Jul-2024

Deactivation of FCC catalysts

A review of technology and expertise to rapidly identify FCC unit catalyst contaminates, hydrothermal factors, and other obstacles to catalyst performance.

Warren S Letzsch
Warren Letzsch Consulting PC

Article Summary

Catalytic cracking replaced thermal cracking for converting heavier molecules in crude oil to smaller, more desirable products such as gasoline and diesel. The heart of the process is the catalyst, which is responsible for the conversion and yields generated during the operation. Catalytic cracking follows a carbenium ion cracking mechanism rather than the less desirable free radical system of thermal cracking. Both reaction mechanisms compete with the conditions found in the cat cracker. The process itself has evolved around the catalysts being developed and used over time. These catalysts deactivate over time in several ways. While some loss of performance is inevitable, it is important to minimise any abnormal loss of activity and/or selectivity.

Hydrothermal deactivation
There are several ways catalysts could lose activity in the FCC unit due to high temperatures in the presence of steam. These are:
•  Elevated individual particle temperatures
•  A higher regenerator mix temperature
•  Repeated reduction/oxidation cycles.

All three are valid mechanisms and do contribute to deactivation.

Fluid catalytic cracking (FCC) was introduced in 1942 and became the preferred design due to its ease in circulating catalyst and removing heat. Synthetic silica-alumina catalysts were developed for the FCC process, and these early amorphous catalysts had an activity proportional to their surface area. These products started with surface areas ranging from 400 to 550 m²/gm and equated with surface areas of 125 to 180 m²/gm. Laboratory tests showed that high temperatures and the presence of steam would cause the loss of activity and surface area. These conditions are present in the regenerator of the catalytic cracker.

Shell¹ conducted experiments with tagged catalyst and found that 80% of the surface area the catalyst lost in the unit occurred in the first 24 hours after the catalyst was added to the unit. Using the regenerator conditions, they found that the loss of surface area could not be duplicated in the laboratory. The catalyst was apparently seeing higher temperatures than those measured. To achieve the observed deactivation, high hydrocarbon and oxygen concentrations were required. Fresh catalyst, with its high surface area, could absorb a lot of hydrocarbons in the small pores, resulting in coke concentrations of up to 5 wt% on freshly added catalyst particles.

Oxygen concentrations of 20% are present where air is introduced to the regenerator or in the spent catalyst transfer lines that use air to convey the catalyst to the regenerator. Figure 1 shows the calculated temperatures more than the spent catalyst/air (oxygen) mix temperature with varying combinations of coke and oxygen. These temperatures (1,550-1,600˚F) are clearly high enough to cause catalyst deactivation.

The catalyst stripper plays a role here, and poor stripping will lead to higher regenerator temperatures. Design parameters of the stripper include catalyst residence time, temperature, steam rate, and hydrodynamics. Short-circuiting of the catalyst needs to be avoided, while good mixing of the steam and spent catalyst is essential.

The burning takes place in milliseconds and is repeated with every cycle through the unit. As the surface area decreases, the rate of deactivation declines since the particles will no longer absorb extra hydrocarbons. These amorphous catalysts were replaced with catalysts containing zeolites in 1962. Within a decade, they were used in every unit and all the FCC reactors were modified to operate with shorter catalyst residence times. The older amorphous catalysts were so inactive that reactors were typically designed for two weighted hourly space velocity (WHSV). They contained as much catalyst as the regenerator. 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 regeneration was developed to reduce the coke on regenerated 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 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 concentrations 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 study3 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 limitations. Pores smaller than 50 Angstroms (Å) can cause capillary 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 metals4, such as sodium and potassium, can cause a severe loss of catalyst performance. Potassium poisoning 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 associated with the zeolite in the catalyst and causes the crystal structure to collapse at lower temperatures. This is analogous 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 structures that boil above 1,100ºF and can act as dehydrogenation catalysts. They also increase delta coke, which increases the regenerator temperature, thus compounding 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 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, passivation 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 vanadium can be at least partially passivated with catalyst formulations 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.