Catalytic Means to Increase FCC Octane Barrels, Part 1.
Increased Zeolite Accessibility Reduces Gasoline Recracking

Introduction: The Value of an Octane Barrel

Since the gasoline produced in fluid catalyst cracking units (FCCU’s) often has sufficient octane to be sold as regular unleaded gasoline, its value can be approximated by the refinery selling price, or "rack price", of unleaded regular. The costs associated with failure to produce gasoline with the 87 road octane specification of regular gasoline and the benefits associated with exceeding the octane spec are represented by the octane barrel cost, where octane barrels are calculated by multiplying the barrels of gasoline times the difference between FCC gasoline octane and unleaded regular octane.

Although the value of an octane barrel depends on refinery economics, it can be estimated from the selling prices of unleaded regular and unleaded premium when the demand for premium gasoline exceeds supply. Under these conditions, the value of an octane barrel is the difference between premium and regular gasoline "rack prices" divided by the octane difference between leaded and premium. At the end of l988, this calculation indicated the value of a road octane barrel was 59 cents. This high value of an octane barrel and the strong demand for premium justify the use of FCC catalysts designed to increase octane barrels.

Catalyst Fundamentals that Affect Octane Barrels

An FCC octane barrel catalyst must increase gasoline octane without adversely affecting yields. We have found that gasoline selectivity and octane will both be increased if the zeolite accessibility uncracked gas oil molecules is increased. Increased zeolite accessibility is achieved in a catalyst with small diameter zeolitic crystals that have been dealuminated to a unit cell size below 24.50 angstroms by a process that creates a "highway access system" of mesopores for the reactant molecules. An active matrix compliments zeolite accessibility by precracking the reactant molecules, reducing their size and minimizing their tendency to form coke in the zeolite pore structure.

FCC gasoline octane barrels also can be improved if the acidity of a cracking catalyst 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 but does not significantly improve 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 within this narrow range "controlled acidity" catalysts.

In this first installment of a two part series on octane barrel catalyst design, the role of zeolite accessibility is explained. Part 2 will describe the concept of controlled acidity in maximizing octane barrels.

Reducing Diffusional Effects Improves Gasoline Selectivity

Mass transfer within the cracking catalyst zeolite has a significant effect on gasoline selectivity. Chemical engineers who study the fundamentals of catalysis have known since the 1950's (Ref. 1) that diffusional effects can reduce the selectivity of chemical reactions for intermediate products. In catalytic cracking, C4 and lighter hydrocarbons are the end products of the reactions. Gasoline and diesel fuel are the valuable intermediate products whose yields can be reduced by diffusional limitations.

Since the early development of zeolitic cracking catalysts, researchers have also known that it was difficult for gas oil molecules to diffuse into the center of zeolite crystals (Ref. 2). This diffusional limitation is caused by the 8 angstrom size of the zeolite pore openings. Since many gas oil molecules are too large to diffuse into the pores, the cracking sites in the crystal interior primarily crack smaller molecules like gasoline. Improving access to the zeolite interior reduces gasoline "recracking" and enhances gasoline selectivity.

The most straightforward way to minimize diffusional limitations in zeolite crystals is to increase the exterior surface of the crystals by reducing their size. For equal weights of zeolite, decreasing the crystal diameter by a factor of three will triple the outer surface area. The effect of zeolite crystal size is demonstrated in a recent study (Ref. 3) where the average zeolite crystal size of an experimental catalyst was reduced from 0.9 microns to 0.3 microns. The experimental results in Figure 1 show the smaller zeolite crystals yield from 1 to 10 weight percent more gasoline.

An alternative to reducing the crystal size of the zeolite is to develop a network of larger pores within the crystal. Pores that are greater than 8 angstroms in diameter will allow gas oil molecules to more easily penetrate to the center of the zeolite. This reduces the amount of relatively inaccessible surface area available for gasoline recracking, promotes gas oil cracking, and improves gasoline selectivity.

Thermal Aging of Zeolite Creates a Network of Pores

It is possible to develop a secondary pore network with pores at least 40 angstroms in diameter by exposing zeolite to elevated temperatures (Ref. 4,5). This thermal aging process results in the collapse of a portion of the zeolite crystal, creating a network of holes and cracks. The collapse of the alumina silicate framework of the zeolite crystals is the result of steam extraction of the aluminum in the framework. The longer a catalyst is aged, the more extensive the secondary pore structure becomes. Figure 2 shows large 140 angstrom diameter channels develop and 40 to 60 angstrom pores double in number as aging is increased (Ref. 4).

Impact of Zeolite Defect Density and Dealumination

Zeolite can be aged or dealuminated either in a commercial unit or during FCC catalyst manufacture. If dealumination conditions are properly controlled during manufacture, silicon atoms from the collapsed zeolite framework will diffuse through the crystal to fill the zeolite defects left by the extracted alumina. This "healing" process strengthens the remaining part of the crystal and minimizes the amorphous silica produced by dealumination. The uncontrolled dealumination in a commercial FCC regenerator results in more silica debris and less crystal stability.

The amount of silica debris can be quantified by an X-ray diffraction (XRD) measurement we call the zeolite defect density index. The silanols formed by aluminum extraction disturb the (XRD) pattern by creating irregularities in the zeolite crystal lattice. This results in a smearing of the sharp XRD pattern that is charateristic of defect free zeolite. The more defects in the crystal, the broader the width of the normally sharp bands in the pattern. BASF uses the line broadening of the XRD pattern to calculate the zeolite defect density index. The lower the defect index, the sharper the X-ray pattern and the lower the amount of silica debris that may form in the crystal. Lower zeolite defect density catalysts enhance selectivity through avoidance of silica debris pore blockage.

Alumina debris is also produced when zeolite is aged in a commercial FCC unit. The alumina extracted from the crystal framework accumulates in the zeolite pores (Ref. 4,5) and obstructs the access of gas oil molecules to the zeolite interior. When zeolite is dealuminated during catalyst manufacture, acid solutions can solubilize and wash away this alumina (Ref. 6,7). Thus zeolite dealumination during FCC catalyst manufacture can minimize both silica and alumina debris, resulting in an unobstructed secondary pore network that maximizes zeolite accessibility.

Gasoline Recracking Reduces Octane

Without an unobstructed secondary pore system, zeolite has poor diffusion characteristics that promote recracking (Ref. 8). The effect of gasoline recracking on research octane and product distribution can be demonstrated by the fresh catalyst addition study summarized in Figure 3. A base operation was first established in a catalytic cracking circulating pilot unit, and then the unit was monitored for 4 hours after 5 weight percent fresh catalyst was added to the circulating inventory. The fresh catalyst addition depressed gasoline selectivity by 2.8 weight percent of conversion, reduced research octane by half a number, increased coke yield, and increased the yield of LPG saturates.

The poor gasoline selectivity and low octane observed above can be explained by the combination of mass transfer and hydrogen transfer properties of the fresh portion of the catalyst inventory, which promotes the formation of coke and LPG saturates from gasoline. A simplified equation to represent these effects is:

Aromatic Gasoline + Gasoline Olefins -----> Coke + LPG Saturates

Gasoline molecules concentrate in the less accessible interior regions of the zeolite. Hydrogen is transferred from the aromatics in the gasoline to olefins in both the LPG and gasoline fractions. The hydrogen transfer causes gasoline aromatics and olefins to condense to form coke. Gasoline olefins also are recracked to LPG. The consumption of both gasoline olefins and aromatics depress octane as well as gasoline yield.

The Role of an Active Matrix in an Octane Barrel Catalyst

The matrix of an FCC catalyst is the support for the zeolite crystals. The matrix pore structure should be accessible to large molecules to assure these molecules have access to the zeolite. An accessible matrix also insures that gas oil molecules can successfully compete with gasoline for matrix cracking sites. This minimizes gasoline recracking and provides octane barrel benefits similar to those provided by accessible zeolite.

An active matrix contains acidic aluminum, but most of these acid sites are too weak to crack light gas oil. Only aluminum sites that are isolated have enough acidity to crack most hydrocarbons, and due to the random arrangement of silicon and aluminum atoms in the matrix these isolated sites are relatively rare. Since there will be few strong acid sites compared to the number of aluminum atoms, the matrix sites that produce gasoline will be widely separated. This will prevent hydrogen transfer that saturates gasoline olefins, improving the octane of gasoline formed on the matrix.

Matrix activity is required to crack large molecules that cannot penetrate the zeolite pores because a catalyst typically has more matrix surface area than external zeolite surface area. FCC catalysts typically have 30 to 100 square meters per gram of matrix surface area at equilibrium conditions. External zeolitic surface area is comparatively small: less than 2 square meters per gram for zeolite with a crystal size of 0.2 microns (Ref. 9). When the large molecules cracked by the matrix are aromatic, aromatic gasoline is produced. This improves road octane barrels.

An active matrix with accessible sites for bottoms cracking also may be necessary to make full use of the zeolite pore structure. A recent study (Ref. 10) used dealuminated zeolite incorporated in an inactive matrix to minimize the influence of matrix on the results. It was found that the large pores of the zeolite could "act as traps for large hydrocarbon molecules." When the acidity on the pore walls was not sufficient to crack these molecules, they condensed to form coke that plugged the pores, hindered diffusion of molecules into the zeolite interior, and reduced gasoline selectivity.

Our data suggests an active matrix prevents heavy hydrocarbon molecules from plugging the zeolite pore structure. When a bed of active matrix material was used to filter a zeolite bed in a MAT cracking experiment, conversion was increased by 18 weight percent over the level when the beds were reversed. Selectivity to gasoline was also substantially improved. This experiment demonstrates that an active matrix can convert heavy hydrocarbons to lighter molecules that will not coke up the zeolite pore structure. The resulting increase in zeolite accessibility improves both activity and gasoline selectivity.

FCC Catalyst Requirements for Increased Octane Barrels

We conclude that octane barrels are increased by small zeolite crystals with an unobstructed secondary pore network that improves zeolite accessibility. When zeolite is dealuminated under properly controlled conditions during catalyst manufacture, a secondary pore structure develops, silica debris is minimized, and a substantial fraction of the alumina that blocks the pores is removed. The secondary pore structure of the fresh zeolite will then be clear from obstructions. Combining this dealuminated zeolite with an active matrix will maintain zeolite accessibility during gas oil cracking by reducing coke formation in the zeolite pores.

References

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