Catalyst Matrix Properties Can Improve FCC Octane & Cetane
The matrix of a FCC catalyst serves both physical and catalytic functions. Physical functions include providing particle integrity and attrition resistance, acting as a heat transfer medium, and providing a porous structure to allow diffusion of hydrocarbons into and out of the catalyst microspheres. The matrix can also affect catalytic selectivity product qualities, and resistance to poisons.
The matrix tends to exert its strongest influence on overall catalytic properties for those reactions which directly involve large molecules.
In this report reference is made at various times to matrix type, as it refers to the matrix surface areas of fresh catalysts. The following summary should be used to correlate matrix type with actual fresh catalyst matrix area measurements:
The subject of this report deals with the effects increased matrix surface area can have on improving FCC octane and cetane. Future reports will discuss the effects of matrix on improving bottoms upgrading and resistance to poisons.
Much of the information contained in this newsletter are excerpts from a technical paper entitled, Matrix Effects in Catalytic Cracking, authored by Messrs. L.D. Silverman, W.S. Winkler, J.A. Tiethof and A. Witoshkin of the BASF Corporation. It was first presented at the NPRA Meeting held on March 23-26, 1986 in Los Angeles, California. The paper, which gives an excellent overview of catalyst matrix effects, can be obtained directly from the BASF Corporation. To obtain a copy, you can either ask your local BASF Technical Sales representative, or call toll free at 800-932-0444, in NJ call 800-624-0818.
Increasing a catalyst's matrix surface area will have a positive effect in upgrading product quality. Gasoline octane increases of 0.5 to 1.5 RON have been commercially demonstrated and attributed directly to increasing matrix surface area. Similarly improvements in cetane index of 1.7 to 2.9 numbers have been gained in commercial units. Increasing matrix surface area within a given family of catalysts can have the drawback of some increase in coke and gas make. Therefore, the optimal situation for a refiner is to take full advantage of matrix cracking up to the limits of his particular unit.
In a commercial operation at refinery B. simultaneous improvements in both the gasoline octane and light cycle oil cetane index were observed as illustrated in Figure 1. At this refinery a rare earth exchanged zeolite catalyst with moderate matrix surface area replaced one with a low matrix surface area. The selective cracking of high boiling range molecules and the reduced hydrogen transfer associated with matrix cracking resulted in a gasoline octane improvement of approximately two RON and a light cycle oil cetane index increase of almost three numbers. The changes in qualities were proportional to the increase in matrix surface area in the circulating inventory. This is a processing advantage since most operational changes that improve octane, such as increased conversion or increased feed aromaticity will have a negative effect on LCO quality and depress the cetane value.
Increases in octane observed in a number of commercial operations are summarized in Table 1. Additional commercial data illustrating improvements in the light cycle oil cetane index gained with increased matrix activity are summarized in Table 2. The data presented in these two tables represents operations where there were no significant changes in factors other than cataIyst matrix (e.g. operating severity, feed changes, zeolite unit cell size or rare earth content).
Note: Octane values are clear Research Octane Numbers corrected to constant reactor temperature and conversion. The catalysts within each case contain zeolite with similar levels of rare earth. Matrix type refers to matrix surface area of fresh catalyst; low <75 m/g, moderate = 75-150 m/g, high >150 m/g.
Note: Comparisions at constant unit conversions and constant LCO cut points. Matrix type refers to matrix surface areas of fresh catalyst; low<75 m/g., moderate = 75-150 m/g., high>150 m/g.
Research studies confirm the product quality improvement from matrix effects seen in commercial operations. The data in Table 3 compares the product qualities from FCC pilot tests on rare earth catalysts with low and medium activity matrices. It can be seen that the gasoline from the higher matrix catalyst has both a higher gasoline octane and a higher olefin content by PONA analysis. Matrix cracking improves gasoline octane because hydrogen transfer is reduced compared to zeolitic cracking.
The pilot data in Table 3 also shows that the catalyst with the more active matrix produced LCO with higher cetane as well as gasoline with higher octane. A significant effect of the active matrix is the preferential cracking of aliphatic materials (paraffin and naphthene molecules or fragments) from bottoms into LCO. This is evident from Nuclear Magnetic Resonance (NMR) data showing a much lower aliphatic carbon content in the bottoms from the catalyst with the more active matrix. The preferential cracking of aliphatic compounds (or fragments) reflects the fact that aromatic structures are very resistant to cracking. As the catalyst with the active matrix cuts deeper into the bottoms at constant conversion, some of the aliphatic compounds cracked from the bottoms end up in the LCO fraction and raise its fuel quality. This is consistent with the higher aliphatic content of the LCO from the active matrix catalyst and the corresponding higher LCO cetane. As can be seen from the pilot data in Table 3, as the cetane of the light cycle oil improves with higher matrix surface area catalyst, the bottoms yield is reduced and the properties of the bottoms are also affected. The higher matrix surface area catalyst produced a more aromatic bottoms with a lower API gravity. Data from commercial operations have verified the result of this study. Improvements of up to a 40% reduction in bottoms yield have been commercially realized when changing from a low to high surface area catalyst.
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