ADDITIVES PLAY IMPORTANT ROLE IN FCC DEVELOPMENT

The need for specialty fluid catalytic cracking (FCC) additives is likely to increase, given the increasing complexity of the refining environment and the arrival of reformulated gasoline prompted by the most recent national environmental laws.

C. W. Kuehler of Chevron Research & Technology Co. U.S.A. covered the participants in the field and the state of FCC additive technology this past June at Akzo Chemie's catalysts symposium in Scheveningen, The Netherlands. Other authors of the paper presented were A. S. Krishna, C. R. Hsieh, A. R. English, and T. A. Pecoraro, all with Chevron.

The FCC process has undergone numerous changes in its 50-year history, Kuehler pointed out.

Fig. 1 shows the impressive continual growth of the process.

One important aspect of additives is their ability to alter and control the nature of the FCC catalyst, thereby controlling the reactions carried out in the reactor and regenerator.

Additives offer a number of advantages, according to Kuehler:

They can be taken in and out of use with relative ease.
Their effects are observed fairly quickly-from a matter of minutes with CO-combustion promoters, to hours or a few days with octane-enhancing additives and SO,,-reduction agents, or to weeks with metals traps.
They avoid the cumbersome task of having to change a complete inventory of catalyst to meet a temporary or seasonal change in unit operating objectives.
They can make the use of relatively expensive materials economical.

An array of additives is now available to refiners, including Y zeolites for gas oil cracking, combustion promoters, SOx-reduction agents, fluidization aids, octane-enhancing additives, nickel passivators, vanadium passivators, bottoms-cracking additives, and novel zeolites.

Some FCC additives are in a solid form, others in liquid or gaseous forms. The typical application involves a solid additive with physical characteristics similar to those of FCC catalyst (dictated by fluidization requirements), or a liquid additive introduced into the FCC riser by direct injection, with or without dispersant.

Table 1 presents a summary of additives and their commercial status.

Chevron has had a longstanding interest in FCC additives over the past 2 decades, says Kuehler.

Significant developments include Transox for SO, reduction, and tin and bismuth compounds for vanadium and nickel passivation, respectively.

More recently, a process was developed that reduces dehydrogenation and uses an improved ZSM-5 additive for octane enhancement and vanadium traps for zeolite protection.

SOLID-PARTICLE ADDITIVES

Solid particle additives in commercial use, or under various stages of development, include CO-combustion promoters, SOx-reduction agents, octane-enhancement additives, vanadium traps, and bottoms-cracking additives.

Except for activity control, refiners typically maintain additive concentrations in unit inventory of less than 5%, and often less than 1%. These additives are essentially inert to the primary cracking process, and act as diluents.

The effectiveness of the additive at relatively low concentrations is critical because higher dosages can cause significant dilution of the host catalyst's function. Furthermore, high concentrations are economically prohibitive.

EQUILIBRIUM CATALYST DILUTION

A recent Chevron pilot plant study evaluated the inventory diluting effects of "inert" additives. Although microactivity test (MAT) data exhibited nonlinear trends, the effect of the diluent on activity was essentially linear in the pilot plant.

In other words, according to Chevron, the activity of the catalyst/diluent blend is the weighted average of the activities of the catalyst alone and the diluent alone, at constant catalyst circulation.

However, coke make is also reduced in a linear fashion for most additives, assuming relatively low surface area diluents.

Therefore, if riser outlet and feed-preheat temperatures are held constant, under heat-balanced conditions, catalyst-to-oil ratio will increase, helping to offset the activity dilution.

Consequently, in units not operating against a catalyst circulation limit, dilution of the cracking catalyst, up to additive concentrations of about 5 wt %, should not have significant effects on unit conversion and cracking selectivity.

CO-COMBUSTION PROMOTERS

Well over 60% of the FCC units in the U.S. employ CO-combustion promoters. A comparison of the different types of promoters available is shown in Table 2.

Combustion promoters can be used to convert an FCC unit from conventional to full CO combustion, or to partial CO combustion. Maximum reductions in carbon-on-regenerated-catalyst, after-burning, and CO and SOx emissions are obtained with complete CO combustion.

In situations where the regenerator temperature limits prohibit the practice of complete CO oxidation, partial CO burn can be achieved with the addition of promoters in conjunction with controlled (low) flue gas excess oxygen.

The benefits of partial CO combustion and the tradeoffs between the benefits of carbon-on-regenerated-catalyst and catalyst-to-oil ratio are widely recognized.

Since the introduction of combustion promoters in the mid-1970s, several improvements have been made, resulting in reduced requirements for maintenance dosages in commercial operation.

The effectiveness of the promoter depends not only on the amount of platinum used, but also on the platinum dispersion, the type of base material employed, and retention characteristics such as attrition resistance and density.

SOX-REDUCTION AGENTS

Catalytic SOx-reduction agents are based on the principle of SO, adsorption in the regenerator and H2S release in the reactor. Most SOx-reduction additives are alumina-based. The exception is Katalistiks' magnesium-aluminate, spinel-based technology, called Desox.

Commercially available additives for SOx reduction are as follows: Kdsox (Akzo), Transcat (Chevron), Additive "R" (Davison), Ultrasox (Engelhard), Lo-Sox (Intercat), and Desox.

Chevron's applications for its Transox process, which employs Transcat, have included regenerators in both complete and partial CO combustion, with and without promoters, hydrotreated and residua feeds, and base SOx levels ranging from 50 to 800 PPM.

OCTANE-ENHANCING ADDITIVES

ZSM-5-containing additives, introduced in 1981, have been shown to boost gasoline octanes in FCC use. The effects of ZSM-5 are fairly dramatic and can be observed in a matter of hours.

A list of ZSM-5-based additives currently offered is shown in Table 3.

VANADIUM TRAPS

Another novel application of catalytic additives in FCC is the use of second-particle vanadium traps, pioneered by Chevron. Development work on this concept began in the late 1970s.

Laboratory tests established that vapor-phase migration of vanadium occurs under regenerator conditions, and that certain materials react with the volatile vanadium species to form stable compounds.

This early work resulted in several patents on calcium and magnesium-based compounds as vanadium-passivating agents. When used as second-particle additives, these materials picked up two to three times more vanadium than the catalyst did, in laboratory tests.

BOTTOMS-CRACKING ADDITIVES

The 650+F. product from the FCCU is the lowest in value. A common objective is to reduce the yield of this material, referred to as decanted oil, slurry oil, or heavy cycle oil (HCO). This is particularly important when heavy gas oils and residua are processed in the FCCU.

One way to minimize decanted oil yield is to employ a catalyst with high matrix activity. However, such catalysts typically produce higher yields of coke and gas, making them unsuitable for certain applications.

Many refiners process residuum on an intermittent basis. As the quantity of residuum fed to the FCCU varies, selective application of a bottoms-cracking additive may provide the refiner added flexibility.

Important additive characteristics are zeolite content, surface area, and silica/alumina ratio. Analyses of a number of additives are shown in Table 4.

Pilot plant results of Chevron's research in this area, in conjunction with a major catalyst manufacturer, are shown in Fig. 2. This study identified statistically significant differences between bottoms-cracking capabilities of base catalysts.

The addition of 15% of a high matrix surface area bottoms-cracking additive (BCA) to one of the less-effective base catalysts illustrates that it is possible to improve the selectivity for light cycle oil (LCO), relative to coke and heavy cycle oil, through the use of additives.

ADDITION SYSTEMS

Solid-phase addition systems have been developed, enabling refiners to add small quantities of specialty additives to the FCC regenerator in a controlled manner.

The most common application of liquid additives in FCC is in passivation of contaminant metals such as nickel and vanadium.

Three different liquid additives are available for passivating the dehydrogenating effects of nickel on cracking catalyst, based on antimony and bismuth compounds, and a third proprietary material (Table 5).

Metallic promoter and SO, sorbent have also been added to FCCUs as an oil or water-soluble dispersible compound to scavenge SO,. Other than the use of steam as a dispersant or stripping agent, the use of gaseous additives has been limited.

Lift gas (consisting Of C3-gas, such as absorber gas) is sometimes used to improve feed/catalyst contacting, and partially passivate catalyst metals.

FUTURE PROSPECTS

The need for specialty FCC additives is increasing, in light of the challenges facing refiners in meeting Clean Air Act specifications.

Chevron sees the application of specialty zeolites to effect unique FCCU chemistry as one area of increasing research. The objective is to selectively enhance the yields of specific products and the type of hydrocarbon species that contribute to product quality improvements.

The patent literature includes references to the use of several novel materials in FCC.

The commercial viability of these new materials remains to be proven.