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Apr-2024

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

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Article Summary

A fluid catalytic cracking (FCC) unit needs to operate at a steady state with the regenerator within acceptable 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 circulation rates reach a limit, then higher reactor temperatures may not be possible. Any unit running at a catalyst circulation limit is losing revenue. High regenerator temperatures can lead to equipment damage, low conversions, and excessive catalyst deactivation.

FCC unit regenerator temperature is a dependent variable that is a consequence of the heat balance. For all ‘cat crackers’, 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:

Œ      Wt% coke          =        Delta coke       x     Catalyst/oil
 or,     lb Coke               =        lb Coke            x     lb Catalyst
          lb Feed                          lb Catalyst              lb Feed

Equation 1 is an identity and applies to all catalytic cracking units. Other equations that come from the heat balances around the unit are:
 Heat to reactor/Hr = lb catalyst/Hr x cp catalyst x Delta T (Regen-Reactor)
Ž Efficiency of regeneration = Heat to reactor / Total heat generated in regenerator
Therefore: Coke burned/lb cat x heat of combustion is proportional to delta T (Regen-Rx)
Or delta coke is proportional to the Delta T (Regen-Reactor)

For a full CO burn regenerator, the regenerator temperature will rise about 40ºF for every 0.1 wt% increase in delta coke. In a partial burn regenerator operation, the temperature difference is about 30-32ºF.
Delta coke is the variable that influences the regenerator temperature since the reactor temperature is set to provide 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:
Œ    Feedstock
        Operating variables
Ž    Catalyst
       Unit design.

Tables 1 and 2 list ways to reduce or increase the regenerator temperature. These are explored more in each of the following sections.
Feedstock

When the feedstock quality changes, the delta coke will usually change. The important feed properties are its composition and its boiling range. The three families of hydrocarbons are paraffins, naphthenes, and aromatics. These tend to increase the coke laydown on the catalyst inversely proportional to their hydrogen content. Paraffins produce low delta cokes, while polynuclear aromatics (PNAs) are coke precursors. The single-ring aromatics end up in gasoline, and the two-ring aromatics go to diesel. The larger aromatics can undergo alkylation and isomerisation reactions that lead to condensation reactions and increased delta coke.

Delta coke will normally change with feedstocks. This is more likely to happen when changing crude oils, which is common in large refineries. Stratification of feeds can occur in tanks if they are not circulated. Both the quality of the feed can change, and the concentration of contaminants like sodium can increase by a factor of 10 from the top to the bottom of the tank. When other streams make up part of the feed going to the FCC unit, such as coker gasoil, feed quality is going to vary due to the changes in the coker gasoil composition and the amount processed. Even if a feed pretreater is in front of the cracker, the feed to the unit will not be uniform.

The boiling range can greatly alter the delta coke. When some vacuum resid is in the feed, at least a portion of the PNAs will lay down as coke, and the nitrogen content will increase. Many nitrogen compounds are basic and form bonds with the acidic sites of the catalyst, reducing its activity and selectivity. More coke is formed since they do not desorb from the acid sites as quickly as other hydrocarbons.

Conradson Carbon (Concarbon) is a test used to measure the coking tendencies of the feed. If the residue is mostly aromatics, the increase in delta coke will be the weight per cent Concarbon divided by the catalyst-to-oil ratio. If the residue is mostly paraffinic, then only a portion (possibly less than half) of the Conradson Carbon goes to coke.

The coke-making tendencies of the feed can increase if the vacuum tower operation deteriorates. There are a number of reasons this can happen, but pushing the distillation column beyond its design limits is a frequent cause. The darker colour of the gasoil is a sure sign that the FCC feed has more coke precursors.

Gasoline was the main product from early FCC units, and diesel was often included in the feed stream. The front end of the straight-run cat feed does not produce as much delta coke as the rest of the feed. The diesel range (430-650ºF) molecules went through the units largely unconverted when amorphous cracking catalysts were used and would lower the regenerator temperature. They were recycled to increase conversion, and this stream was called light cycle oil (LCO). Zeolite catalysts will readily crack these molecules. Recycling LCO may lower the coke-making tendencies of the feed. However, if this stream is very aromatic (di-aromatics), the recycle may lead to increased bottoms yield and have only a marginal impact on the regenerator temperature.

Recycle of decant oil, the material taken from the bottom of the main fractionator, is used to increase the regenerator temperature in many units. This is common in cracking units with feed hydrotreaters or those that process high API crudes. The high concentration of multi-ring aromatics in this stream always makes it a variable that can be used to increase the delta coke and regenerator temperature. Low regenerator temperatures may leave more coke on the regenerated catalyst or prevent the complete burning of CO to CO2.


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