I.FLUID CATALYTIC CRACKING: www.wissenschaftler-avh.in
B.CATALYST/ADDITIVES(Contd.)
Q-49:What are the Catalyst Requirements for Increased Octane Barrels in FCC?
A-49:
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.
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 . 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 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.
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
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 zeolite pores..
B.CATALYST/ADDITIVES(Contd.)
Q-49:What are the Catalyst Requirements for Increased Octane Barrels in FCC?
A-49:
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.
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 . 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 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.
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
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 zeolite pores..
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