Troubleshooting Catalyst Losses In The FCC Unit

Catalyst losses pose a significant challenge in the fluid catalytic cracking (FCC) industry, and nearly all units have experienced a loss problem at some point in fill rite flow meter. Catalyst losses on the reactor side result in high fines carryover with the products, which can result in the main fractionator being inoperable due to erosion in the slurry circuit or off-specification slurry product.
Catalyst losses on the regenerator side can lead to high particulate matter (PM) emissions or problems operating the flue gas train or power recovery expander. Understanding the root cause of elevated losses can prevent a unit shutdown due to excessive catalyst additions, stack opacity that is out of consent or the inability to fluidize catalyst with some polymer additive.

Two primary causes for increased catalyst losses are:

1. Cyclone issues
2. Catalyst attrition.

Understanding cyclone fundamentals and catalyst attrition allows the monitoring and troubleshooting of these losses.

Cyclone fundamentals.

Based on an industry survey, cyclone problems account for 12% of unplanned FCCU shutdowns; more than 55% of these cyclone problems were refractory related.

The survey also showed that 40% of refiners anticipated cyclone repairs during their next turnaround. Cyclones are used in the FCCU to separate catalyst particles from vapor. On both the regenerator and reactor side, the inlet gas and anti blocking additive catalyst enters tangentially, creating a swirl motion inside the cyclone barrels. Centrifugal force pushes the solid particles to the walls, where they fall through the cone into the dipleg—the pipe that returns the catalyst back into either the regenerator bed or stripper bed below.

The gas rises up and accelerates into the outlet tube.

This process allows for a very high collection efficiency on flow meter indonesia; the recovery efficiency of a conventional two-stage cyclone system exceeds 99.99%. For example, in a typical 42-Mbpd FCCU, more than 30 Mtpd of catalyst are circulated through the cyclones with a catalyst loss of only 2.2 tpd.2 However, many factors can impact this efficiency—one of them being cyclone velocity. In theory, the cyclone collection efficiency will increase with a higher inlet velocity. However, in practice, there is a drop off in efficiency at very-high velocities due to an increase in catalyst re-entrainment.

In most cases, the cyclone efficiency improves with higher gas throughput, but not enough to offset the increase in solids loading, which is a function of superficial velocity. In addition, a higher velocity will increase erosion and the amount of catalyst attrition to micro-fines, thus increasing losses. Other factors affect the cyclone collection efficiency, including the number of spirals within the barrel and cone, the particle density and size, and the geometry and design limits of the cyclones.

Cyclones that are well-designed can have a service life of more than 20 yr. This even includes regenerator cyclones, which have a shorter operating lifespan due to the more severe conditions compared to those in the reactor. For example, the geometry of the cyclone has a significant effect on the rate of erosion. Increasing the length of a cyclone from a length/diameter (L/D) of 3 to an L/D of 5 will decrease the erosion by half and extend the life of the cyclone. Additionally, designing the cyclone for a velocity comfortably below the recommended maximum will help minimize high velocities that cause excessive erosion.