Catalytic Means To Increase FCC
Octane Barrels, Part 2.
Controlled Acidity Reduces Gas Yields Without Octane Debit
The value of gasoline produced in fluid catalyst cracking units (FCCUs) can be approximated by the "rack price" of unleaded regular. The octane barrel value is an adjustment to the gasoline price that represents the costs associated with failure to produce gasoline on specification road octane and the benefits associated with exceeding the octane spec. Octane barrels are calculated by multiplying the barrels of gasoline times the difference between FCC gasoline octane and unleaded regular octane. We define an octane barrel catalyst as a catalyst that increases the number of octane barrels.
According to our definition, an octane barrel catalyst must increase gasoline octane without adversely affecting yields. Improved zeolite accessibility improves octane barrels by reducing recracking reactions promoted in the interior of the zeolite crystal. Recracking of gasoline has the empirical formula: Gasoline olefins plus gasoline aromatics yield coke plus LPG saturates. Reducing this recracking increases the yield of high octane gasoline, reduces the isobutane to olefin ratio in the light hydrocarbons produced, and lowers coke yield.
An active matrix contributes to high gasoline selectivity by protecting the zeolite pore structure from coke formation. An additional benefit of cracking these heavy molecules on the matrix is the conversion of heavy fuel oil to diesel and other more valuable products.
Octane barrels also can be improved in a gas limited FCCU if the cracking catalyst acidity is controlled so that unit cell size equilibrates to approximately 24.30 angstroms. Hydrogen transfer data indicates that reducing unit cell size below 24.30 increases gas yield without significantly improving octane. The combination of high gas make if unit cell size equilibrates below 24.30 and high coke make for fresh catalyst with a cell size above 24.50 indicates that a narrow band of equilibrium catalyst cell sizes is required to maximize gasoline octane barrels. We call catalysts with fresh and equilibrium cell sizes within this narrow range "controlled acidity" catalysts.
This second installment of a two part series on octane barrel catalyst design, explains the role of controlled acidity in maximizing octane barrels. Part 1, described the effects of matrix and zeolite accessibility on catalyst performance.
Higher Acid Strength Increases Octane but Lowers Gasoline
Although zeolitic diffusional resistances have long been known to affect FCC yields, recent cataIyst development work has concentrated on the effects of zeolite acid strength. This property of zeolitic cracking catalysts results in a trade-off between gasoline yield and octane. Pine and co-workers (Ref. 1) demonstrated both octane and gasoline selectivity can be correlated with a single parameter that is related to the zeolite acid strength: the unit cell size.
The unit cell size is a measure of the physical size of the alumina silicate units that form the framework of the zeolite crystal. It is also a measure of the concentration of aluminum atoms in the zeolite framework because aluminum atoms are slightly larger than the silicon atoms. The larger size of the aluminum atoms increases unit cell size at higher aluminum levels.
The acid sites in cracking catalysts are associated with the aluminum atoms. The highest acidity aluminums are surrounded by silicon and oxygen atoms. Low unit cell sizes provide the high zeolite silicon levels needed to isolate the aluminum atoms to form strong acid sites.
The strength of the acid sites is responsible for increasing gas yields (Ref. 1,2,3), and high gas yields adversely affect gasoline selectivity. However the isolation of the acid sites prevents bimolecular hydrogen transfer reactions that saturate olefins produced by catalytic cracking. Since olefins in the gasoline improve octane, reducing unit cell size increases FCC gasoline octane at the expense of gasoline selectivity.
Unit Cell Size Should Equilibrate Above a Minimum Level
Calculations based on a random distribution of aluminum atoms in a zeolite crystalline framework show that the minimum distance between sites increases rapidly as the unit cell size is reduced below 24.45 Angstroms. In Figure 1, we show that the distance between sites is expected to be 16 angstroms if the unit cell size is reduced to 24.30 angstroms. A distance of 16 angstroms is twice the size of the 8 angstroms zeolite pores and is therefore larger than the size of the gas oil molecules that penetrate the zeolite crystal. A larger distance between adsorbed molecules is not needed to prevent them from interacting. Thus bimolecular reactions like hydrogen transfer should be minimized at 24.30 unit cell size and a further reduction in unit cell size should not significantly affect these reactions.
Coke formation has been described as a condensation reaction of aromatics accompanied by hydrogen transfer (Ref. 4), so we would expect that coke selectivity would be reduced at low unit cell sizes where the acid sites are far enough apart to prevent hydrogen transfer. A recent article (Ref. 5) states that a unit cell size of 24.33 angstroms is small enough to minimize coke selectivity. Coke selectivity was reduced as equilibrium unit cell size was lowered from 24.60 to 24.33 angstroms, but further unit cell size reduction did not improve the coke selectivity.
Hydrogen transfer reactions are also responsible for the saturation of gasoline olefins, a reaction that reduces research octane. As Figure 2 indicates, we observed only a marginal decrease in hydrogen transfer and we expect little octane improvement below 24.30 angstroms unit cell size. However, the stronger acid sites associated with smaller cell size will promote gasoline recracking. It is, therefore, best to design an octane barrel catalyst so its equilibrium unit cell size is as close to 24.30 angstroms as possible.
Commercial Operations Have a Unit Cell Size Distribution
The zeolite unit cell size does not remain constant as FCC cataIyst ages. The high temperature, steam rich environment in a fluid catalytic cracker's regenerator promotes the loss of aluminum from the zeolite framework. Laboratory experiments (Ref. 6) where cataIyst aging is accelerated by raising both the steam concentration and the steam temperature show the loss of aluminum is relatively rapid. Under the 1450 degree F steaming in Figure 3, most of the dealumination occurs within the first hour. However, the equiva-lent time of commercial deactivation represented by that hour of accelerated aging is approximately five days.
During its aging time, the high unit cell size catalyst contributes a disproportionately high fraction of the total activity in the unit. Suppose the average catalyst age in the unit is 50 days and that the average catalyst properties are represented by ten hours of accelerated aging. Then Figure 4 shows the activity of the freshest catalyst is more than three times the average activity.
The activity versus time relationship in Figure 4 can be combined with the first order catalyst distribution typical of a commercial catalytic cracker. Integrating activity over the age distribution allows the calculation of the fraction of the total activity contributed by catalyst in each range of ages. Figure 5, the results of the calculation, shows catalyst with one half hour of accelerated aging contributes 15% of the total activity.
Significance of the High Unit Cell Size Catalyst in a FCCU
Since the high unit cell size fraction of the zeolite contributes a disproportionate percentage of the activity in a commercial unit, its hydrogen and mass transfer properties affect a significant fraction of the gasoline. The hydrogen transfer reactions promoted by the fresh catalyst saturate high octane olefins, so high unit cell size fresh catalyst is expected to depress octanes in at least 15% of the gasoline product. In addition to the negative effects on octane the high unit cell size portion of the inventory also catalyzes gasoline recracking.
The effect of gasoline recracking caused by high unit cell size catalyst can be demonstrated by the fresh catalyst addition study discussed in Part 1. Data was presented that demonstrated 5 weight percent fresh catalyst added to the circulating unit inventory depressed gasoline selectivity, reduced research octane by half a number, increased coke yield, and increased the yield of LPG saturates. A simplified equation to represent these recracking reactions is:
Optimal Zeolite Acidity for Octane Barrel FCC Catalysts
The unit cell size distribution of cracking catalyst in the inventory of a commercial unit will vary from that of fresh catalyst to an equilibrium value that is influenced by rare earth content, sodium level and unit operating conditions such as regenerator temperature. Test results demonstrate that the high unit cell size of fresh catalyst will depress gasoline selectivity by making coke and LPG. The same reactions that make coke and LPG simultaneously depress octane. It is thus desirable to use a fresh catalyst with a unit cell size less than 24.5 angstroms.
Data on catalyst hydrogen transfer potential indicates that a unit cell size below 24.30 angstroms does not significantly improve octane, but lower unit cell size zeolite can have high acidity that promotes gas formation. The acidity that maximizes gasoline octane without unnecessarily increasing gas make, therefore, occurs at approximately 24.30 angstroms unit cell size.
Investigation of zeolite acidity effects on octane barrel production thus demonstrates that a narrow band of equilibrium catalyst acidity is best. Fresh catalyst unit cell size should be as low as possible, preferentially less than 24.5 angstroms. Catalyst properties should stabilize the unit cell size at 24.30 angstroms. This narrow range of equilibrium cell sizes is best for gasoline selectivity while only marginal improvements in octane can be obtained by allowing equilibrium unit cell size to stabilize at unit cell sizes less than 24.30 angstroms.
Gas Limited Units Require Controlled Acidity
We have conducted laboratory and computer process simulation studies to provide guidelines for optimizing catalyst acidity by adjusting rare earth content. The results of these studies are summarized in Figure 6. We found rare earth should be adjusted so the catalyst equilibrates to a unit cell size of about 24.30 angstroms to increase octane barrels in FCC units that are at their gas handling limit.
Units that have plenty of gas compressor room may find it beneficial to use a catalyst that equilibrates to a minimum unit cell sizes because this produces a small octane gain. The regenerator temperature limited case and the LPG limited case in Figure 6 show maximum product value when a zero rare earth with a minimum equilibrium unit cell size was used. Both coke make and LPG yield are determined by hydrogen transfer reactions that are practically eliminated when the cell size is reduced to 24.30 angstroms. A further cell size reduction will not change LPG yield or regenerator temperature, which is determined by coke make. Thus reducing unit cell size from 24.30 angstroms to a lower level does not affect conversion in units with regenerator temperature or LPG handling limits. In these cases, octane barrels increase as unit cell size is reduced until the unit reaches a gas handling limit.
1. Pine, L.A., Maher, P.J., and Wachter, W.A., Journal of Catalysis 85, 466 (1984).
2. Corma, A., Planelles, J., and Tomas, F., Journal of Catalysis 94, 445 (1984).
3. Planelles, J., Sanchez-Marin, J., Tomas, F., and Corma, A., Journal of Molecular Catalysis 32, 365 (1985).
4. Gates, B.C., Katzer, J.R., and Schuit, G.C.A., "Chemistry of Catalytic Processes", McGraw-Hill, New York (1979).
5. Rajagopalan, K., and Peters, A.W., Journal of Catalysis 106, 410 (1987).
6. Leuenberger, E.L., Moorehead, E.L., and Newell, D.F., paper AM-88-51, N.P.R.A. Annual Meeting, San Antonio, TX, Mar. 20-22, 1988.