Oct-2001

Overcoming constraints to maximise FCCU feed rates

A discussion of the typical restraints encountered in FCC operation and the methods available to alleviate them, specifically through changes in operation or catalyst, or both

John Black and Jon Petrunia, KBC Advanced Technologies

Article Summary

In today’s market of very low or even negative refining margins, the FCC unit must provide the highest obtainable profit for the refinery. This usually requires operating the unit against two or more constraints simultaneously. In this article, the typical FCC constraints are described and methods discussed to reduce these constraints through operational and/or catalyst changes. Since methods that will reduce one constraint will impact on other constraints, a robust process simulation model is necessary to determine the optimum operating strategies to maximise profit while operating under multiple unit constraints.

Due to the very competitive market conditions, refiners must ensure FCC operations produce the highest possible revenue. The FCC unit has significant impact on refinery economics since it provides a significant portion of the total gasoline pool, and the refinery benefits from the volume expansion that occurs during the cracking process. FCC units are usually operated at maximum charge rates under various types of constraint.

Compressor limitation
A fairly common constraint of the FCC unit is a wet gas compressor limitation. Under this condition, the wet gas compressor will be operating at maximum capacity and no additional dry or wet gas can be processed for recovery. The refiner can reduce the wet gas yield by decreasing unit conversion. This will allow for the addition of more feed until the quantity of wet gas is again at the maximum rate.

The refiner can continue to reduce conversion to allow for additional feed. This swapping of conversion for increased feed is continued until a target conversion level is obtained. There are several strategies available to the refiner to reduce conversion. The most common method is to reduce riser outlet temperature. Another strategy is to reduce conversion by decreasing the equilibrium catalyst activity through reduced fresh catalyst additions. Both of these methods will lower conversion and wet gas yields.

Minimising the amount of thermal cracking that occurs in the riser will also reduce the dry gas components of the total wet gas. Recycle streams produce higher amounts of gas and coke as compared to fresh feed. Decreasing or eliminating slurry and/or heavy cycle oil recycle streams will result in lower dry gas and coke yields.

Minimising the hydrogen yield due to metals contaminants can be achieved by the use of passivating agents for nickel and vanadium. The benefits of metals passivation have been well documented and have been commercially demonstrated. Utilising metals passivation for relatively low levels of nickel on equilibrium catalyst of 1000ppm or less can be economical, if it allows for the processing of additional fresh feed. Vanadium will catalyse hydrogen production at about 20 to 25 per cent of the rate as an equal amount of nickel.

If wet gas is limiting unit charge rate, passivation of vanadium metals of 1500ppm or even less may be economically justified through the reduction in hydrogen yield. This is in addition to any benefits from the improved equilibrium catalyst activity due to vanadium passivation. However, the passivation benefits of vanadium are not as immediately quantifiable as nickel.

Unlike nickel, only the freshly deposited vanadium will be passivated. This requires the entire unit equilibrium inventory to be changed out before the full benefit of vanadium passivation can be realised. The lack of an immediate response to vanadium passivation has caused more than one refiner to discontinue the use of vanadium passivator prematurely.

If the feed nozzles do not utilise current nozzle technology, increasing riser steam may be beneficial as a means to reduce wet gas and coke yields. Increasing riser and feed steam can provide better feed atomisation which normally leads to an improvement in catalyst and oil mixing and less thermal reactions in the riser. The benefits on a specific FCC unit can be determined by monitoring the regenerator temperature and also dry gas yields while increasing dispersion and riser steam. If the increased steam does not improve the coke and dry gas yields, then the steam rates can be reduced. The rate is decreased step-wise until there is an adverse regenerator temperature response.

Improved feed atomisation
Increasing feed preheat can also improve the atomisation of the feed by lowering the viscosity of the oil. This allows for smaller droplet size formation by the feed nozzles. Injecting condensate or naphtha in the riser can also improve feed dispersion. Methods that reduce regenerator bed temperature will also typically cause lower dry gas yields. This is due to a lower mix temperature at the base of the riser.

For units that have elevated feed nozzles, using lift gas instead of steam can produce lower dry gas yields, especially for high metals operations. Depending upon the specific unit, reducing feed preheat may cause either a reduction or an increase in wet gas yields. If the unit has an efficient stripper, the regenerator temperature will increase with increasing feed preheat.

The higher regenerator temperature with increased feed preheat may cause slightly higher dry gas yields. This is due to the higher catalyst/oil mix temperature at the base of the riser and the resulting increase of thermal cracking. If the unit has a poor stripper or an overloaded stripper, the regenerator temperature can remain fairly constant or actually decrease as feed preheat is increased.

This unit will give lower dry gas yields with increased feed preheat. This is due to a lower catalyst circulation rate at the higher feed preheat temperature. The refiner will have to conduct test runs to determine the particular unit’s response to changes in feed preheat and how the stripper responds to changes in catalyst circulation rate.

There are several potential catalyst formulation changes that can reduce wet gas yields. The first is to increase the rare earth content of the catalyst by increasing the amount of rare earth exchange on the zeolite. The higher rare earth catalyst will catalyse more hydrogen transfer reactions. This will result in a gasoline that is more stable and less susceptible to the secondary cracking reactions that produce wet gas.