Catalysis 2024 Issue

Troubleshooting low or high regenerator temperatures

A review of available resources, technology, and expertise to rapidly identify FCC unit regenerator operational problems, including thermal and catalytic effects

Warren Letzsch Refining and FCC Consultant

A fluid catalytic cracking (FCC) unit needs to operate at a steady state with the regenerator within accept - able temperature limits. If the temperature is too low, the reduction in the coke burning rate may cause the coke on regenerated catalyst to rise to a level that impedes both conversion and product yields. If the higher catalyst circu - lation rates reach a limit, then higher reactor temperatures may not be possible. Any unit running at a catalyst circula - tion limit is losing revenue. High regenerator temperatures can lead to equipment damage, low conversions, and exces - sive catalyst deactivation. FCC unit regenerator temperature is a dependent variable that is a consequence of the heat balance. For all ‘cat crack - ers’, the fundamental relationship between the coke make on feed (wt% coke) and the coke burned in the regenerator (delta coke), or (coke on the spent catalyst minus the coke on the regenerated catalyst) is:

 Heat to reactor/Hr = lb catalyst/Hr x c p catalyst x Delta T (Regen-Reactor)  Efficiency of regeneration = Heat to reactor / Total heat generated in regenerator Therefore: Coke burned/lb cat x heat of combustion is pro - portional to delta T (Regen-Rx) Or delta coke is proportional to the Delta T (Regen-Reactor) For a full CO burn regenerator, the regenerator tempera - ture will rise about 40ºF for every 0.1 wt% increase in delta coke. In a partial burn regenerator operation, the tempera - ture difference is about 30-32ºF. Delta coke is the variable that influences the regenerator temperature since the reactor temperature is set to pro - vide the optimum yields and product properties. The regen temperature is the dependent variable that fluctuates with delta coke. The four factors that influence delta coke are:  Feedstock  Operating variables  Catalyst  Unit design. Tables 1 and 2 list ways to reduce or increase the regen - erator temperature. These are explored more in each of the following sections.

Wt% coke

= =

Delta coke

x Catalyst/oil x lb Catalyst



or, lb Coke

lb Coke

lb Feed

lb Catalyst

lb Feed

Equation 1 is an identity and applies to all catalytic crack - ing units. Other equations that come from the heat balances around the unit are:

Methods to reduce the regenerator temperature

Methods to increase the regenerator temperature

1 Reduce feed temperature 2 Minimise slurry recycle 3 Get conversion with reactor temperature vs cat activity 4 Increase dispersion steam, stripping steam, and/or lift gas rates

1 Increase feed temperature 2 Recycle slurry or heavy cycle oil 3 Increase cat activity 4 Use higher delta coke producing catalyst 5 Reduce cat cooler rates 6 Remove light feed streams 7 Increase CO burn 8 Add oxygen to air 9 Remove water from feed 10 Add resid to feed 11 Reduce dispersion steam and/or lift gas 12 Reduce stripping steam (not recommended)

5 Use metals passivators 6 Minimise CO combustion 7 Use lower delta coke catalyst 8 Add lighter feed

9 Add water to reactor 10 Add feed hydrotreating 11 Add heat removal

Table 2

Table 1

Catalysis 2024