Octane catalysts

The benzene content of FCC gasoline is typically in the range of 0.6 vol /i to 1.3 vol%. CAAA s Simple Model requires RFC to have a maximum of 1 vol% benzene. In California, the basic requirement is also 1 vol% however, if refiners are to comply with averaging provisions, the maximum is 0.8 vol%. Operationally, the benzene content of FCC gasoline can be reduced by reducing catalyst-oil contact time and catalyst-to-oil ratio. Lower reactor temperature, lower rates of hydrogen transfer, and an octane catalyst will also reduce benzene levels. 

Mechanisms of Product Yield and Selectivity Control with Octane Catalysts... 

Two catalytic reactions appear to control the product distribution encountered with octane catalysts hydrogen transfer (H-t) and cracking (C). Both occur simultaneously and control of their ratio (H-t/C) gives the product distributions observed. Generally these reactions may be illustrated as follows ... 

A naphthene is used for this illustration as we believe that the relative amounts of naphthene cracking versus hydrogen transfer control product distributions and qualities in octane catalyst systems. Gasoline selective catalysts favor hydrogen transfer reactions with these molecules with consequent formation of coke. 

In the case of "non-octane catalysts, H-transfer is virtually complete and the reaction proceeds (through aromatic condensation reactions) to coke. 

Competitive USY-Containing Octane Catalyst Calculated. Reference 5a. 

There are three different kinds of octane catalysts in current use. Some are based in part on an active non-zeolite matrix composed of a porous silica/alumina component. Others are based on low cell size (2.425-2.428 nm) ultra stable faujasite (USY), a catalyst composition developed in 1975 (2) for the purpose of octane enhancement. A third catalyst system makes use of a small amount (1-2%) of ZSM-5 as an additive. While the net effect in all cases is an increase in the measured octane number, each of the three catalytic systems have different characteristic effects on the composition and yield of the gasoline. The effects of the ZSM-5 component on cracking is described in other papers of this symposium and will not be discussed here. 

Toluene / 50% Xylene mixture) on research octane (clear) of an FCC gasoline from a USY type of octane catalyst. 

Isomerization of paraffins using current octane catalysts under current conditions is favorably away from equilibrium. Additional isomerization activity would make more normal paraffins and a lower octane at FCC temperatures. A much more olefinic gasoline is a possibility. However, additional olefins above the current olefin levels of 10-30% would have decreased effectiveness, especially on the motor octane number. 

Octane Catalyst Ni + V Coke Yield Regenerator Coke Processed ... 

Catalyst Changeover. Metals Content. The equilibrium sample of a USY octane catalyst, Catalyst A, was withdrawn from an Amoco FCU. Results of metals analyses on the equilibrium catalyst and on its parent are given in Table I. Catalyst A was introduced to the unit during a five-month period over Catalyst B, which was identical to Catalyst A in all respects, except for a low level of contaminant rare earth. Catalyst B had, in turn, been introduced over a rare earth-containing catalyst, Catalyst C, eight months prior to withdrawal of the equilibrium sample. Catalyst history and rare earth contents are summarized in Table II. 

Catalyst B--USY octane catalyst with low levels of rare earth contamination (0.5 wt%). 

During the five months of operation with the zero rare earth octane catalyst, the effective fresh catalyst addition rate, after correction for catalyst loss from the unit as fines, was about 5 tons/day. Based on a rare earth material balance (Table II) that was used to give the best estimate of pedigree, the equilibrium sample consists of 88% USY octane catalyst. The remaining 12% should be a mixture of the prior two catalysts, the first of which contains a contaminant rare earth level of 0.5 wt% versus 0.1 wt% for the octane catalyst. The balance of this mixture is the rare earth-Y catalyst from the previous changeover which exhibits a rare earth level of 0.85 wt% (Table II). 

Coking of Equilibrium Catalyst. A 200 g portion of equilibrium USY octane catalyst (Catalyst A) was precoked in a fluidized bed using a stream of isobutene diluted with nitrogen gas (400 ml/min) at 510°C. The catalyst was held in a 2-inch diameter quartz tube while 27.8 g of isobutene gas was passed upflow through it at atmospheric pressure over a period of 25 minutes. 

USY Catalysts. USY catalysts are advertised by catalyst vendors, as low coke/high octane catalysts. This behavior results from the smaller zeolite unit cell size due to dealumination (21.221. The controlled dealumination leads to fewer but stronger acid sites resulting in increased cracking relative to H-transfer. The decrease in the extent of the exothermic H-transfer reactions also results in net increase in the endothermic heat of cracking for USY catalysts (211. 

Metals passivation compliments the latest generations of FCC catalysts octane catalysts based on USY zeolite technology and chemical dealumination. Octane catalysts equilibrate at lower unit cell sizes, resulting in minimization of hydrogen transfer reactions (15). Commercial tests have demonstrated that antimony does not affect the zeolite unit cell size (9). 

Three Engelhard FOC catalysts, identified as catalysts A, B, and C representing typical gasoline, partial octane and octane catalysts were prepared for comparing various test methods. Chemical and physical properties are presented in Table 1. 

Critical parameters to simulate are the ratio of Catalytic to Thermal cracking and the related chemical composition of the gasoline fraction. Obviously, these parameters are particularly important for the research into FCC product properties and, for instance, for the evaluation of potential octane catalysts. 

The equilibrated unit cell size is well related to the Si/Al ratio, and is a convenient tool in selecting and controlling these octane catalysts. 

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