Reduced Unit Cell Size Catalysts Offer Improved Octane for FCC Gasoline
The increasingly restrictive lead phasedown which commenced on January 1, 1986, and the subsequent increased percentage of unleaded gasoline in the total gasoline pool are forcing many U.S. refiners to analyze their current Fluid Catalytic Cracking unit operations as they search for ways to increase octane. The use of Ultrastable Y Sieve (USY) catalysts is a recognized way to achieve higher octane performance products from fluid cracking units. These USY catalysts have a lower unit cell size than conventional gasoline catalysts. BASF is the leading manufacturer of a variety of USY catalyst series that give a refiner the opportunity to use the catalyst optimally matched for his specific operation.
Definition of Unit Cell Size
The definition of unit cell size is the length of one edge of a theoretical cubic solid of the faujasite zeolite repeating unit. The unit cell size is measured in Angstroms and is a measure of the ratio of silicon to aluminum within the zeolite crystal structure. Because aluminum is the larger of the two ions, the unit cell size dimension gets smaller as the aluminum is removed from the zeolite structure. Reducing the aluminum content of the zeolite will increase the silica to alumina ratio and result in fewer, but stronger and more widely separated acid sites per unit cell. This affects the catalyst's activity and selectivity and ultimately the octane properties of the cat cracked gasoline.
Reduced Unit Cell Size at FCC Equilibrium Conditions Affects Catalyst Performance
All catalyst at equilibrium conditions have a reduced unit cell size when compared with the fresh catalyst state. Reducing the unit cell size stabilizes the zeolite while removing alumina from the zeolite structure. The lower unit cell size of equilibrium catalyst correlates with improvements in research and motor octane. When the silica to alumina ratio is increased there are fewer acidic sites per unit cell, and this inhibits hydrogen transfer reactions that reduce gasoline octane. These undesirable reactions convert higher octane olefins to lower octane paraffins as illustrated by the reaction mechanism shown below:
3CnH2n + CmH2m -> 3 CnH2n+2 + 3CmH2m-6
Gasoline Olefins + LCO Naphthenes -> Gasoline Paraffins + LCO Aromatics
Olefins have higher RON and sometimes higher MON than their analog paraffin counterparts as illustrated by the following examples:
Figures 1,2,3 & 4 respectively show the relationship between unit cell size (A) and research octane, motor octane, wt% C3- gas make, and MAT conversion. This study demonstrates that reduced unit cell size correlates with research and motor octane improvement. However, as these same figures show, one penalty for this octane improvement is some increase in C3- gas make. Another trade-off for the octane improvements that corresponds with a reduction in unit cell size is some loss of catalyst activity. These figures were presented in a paper published by the Journal of Catalysis entitled, "Prediction of Cracking Catalyst Behavior by a Zeolite Unit Cell Size Model". The paper which was authored by Messrs. Pine, Maher and Wachter of Exxon Research and Development Laboratories, Baton Rouge, Louisiana is an excellent reference on the subject.
Benefits of Reduced Unit Cell Size Fresh Catalyst
The data presented in the aforementioned figures are for fresh steamed catalysts. The unit cell size ranges from about 24.25 to 24.32 A and is significantly lower than the unit cell size measurements of fresh catalysts. These figures thus illustrate the effects of alumina removed from the zeolite structure that occurs under equilibrium conditions on an FCC unit.
In the production of FCC catalysts, most fresh catalysts start off with a relatively high unit cell size in the range of 24.50 to 24.75. It is the hydrothermal environment of the FCC unit regenerator which extracts alumina from the zeolite structure, thus, reducing the acid site density and unit cell size of the equilibrium catalyst. The final equilibrium level cell size is dependent upon the rare earth and sodium level of the zeolite. The lower the rare earth and sodium level of the fresh catalyst, the lower the resultant equilibrium cell size and the greater the octane. This phenomena is illustrated by the examples of commercial and lab data below:
One of the keys to optimizing the performance of a fresh FCC octane catalyst is to manufacture a catalyst with adequate fresh catalyst activity and stability while minimizing the use of rare earth. This can be accomplished through the use of a pre-reduced cell size zeolite to provide stability and activity maintenance.
Fresh non-rare earth catalysts which have not had their unit cell size pre-reduced can experience an excessive drop in activity when subjected to high temperature operation as illustrated in Figure 5.
BASF Ultrastable Y Sieve Catalysts Offer Optimum Flexibility & Performance
Because of the above mentioned reasons, the optimum mix of ultrastable Y zeolite and rare earth will depend upon each individual refiner's economics. The value placed on the conflicting demands for octane versus higher activity, lower gas make and reduced gasoline yields will determine the optimum design for an octane catalyst. For this reason BASF has been a leader in providing refiners a choice from a variety of catalysts designed with varying levels of ultrastable zeolite, and rare earth addition. BASF's DYNAMICS Technology in particular offers maximum flexibility for optimizing operating efficiency and product yields. Matching the correct amount of ultrastable Y zeolite for improved octane with the correct amount of rare earth for stability and gasoline yield is possible because of the diversity of ultrastable Y sieve catalysts which BASF has to offer.