Increased Catalyst Matrix Surface Area Improves Bottoms Upgrading
In a previous Catalyst Report, data was presented which showed that increasing a catalyst's matrix surface area will have a positive effect on upgrading product quality. Gasoline octane increases of 0.5 to 1.0 RON have been attributed directly to increasing matrix surface area. Similarly improvements in cetane index of 1.7 to 2.9 numbers have been commercially demonstrated.
The subject of this report addresses the impact increased matrix surface area can have on improving FCC bottoms upgrading. A future report will discuss the positive effect increased matrix can have on improving resistance to feed poisons and contaminants.
The matrix of an 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:
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 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 BASF Corporation. To obtain a copy, you can either ask your local BASF Technical Sales representative, or call 800-932-0444, in NJ call 1-800-624-0818.
The FCC catalyst matrix plays a significant role in determining the catalytic performance as well as affecting heat transfer in the FCC unit. Catalyst matrix also imparts important physical properties to the catalyst. Because large feed molecules cannot readily diffuse into zeolite pores, primary cracking reactions on the catalyst matrix make a major contribution to upgrading bottoms to light cycle oil.
It has been commercially demonstrated that the upgrading properties of catalysts with active matrices can reduce FCC unit bottoms yields by as much as 40%.
At Refinery A, a high matrix surface area catalyst, replaced a low matrix surface area catalyst. As illustrated in Figures 1A, the bottoms yield from this unit decreased (bottoms selectivity improved) in direct proportion to the increase of the high matrix surface area catalyst in the circulating catalyst inventory. Figure 1B shows that reduction in bottoms yield was evident over a wide range of conversions. The decrease in the bottoms resulted in an equal volumetric increase in the light cycle oil yield.
A review of the five commercial operations summarized in Table 1 shows that improvements in bottom upgrading was achieved even in a situation where a refiner switched from a low matrix surface area catalyst to one with a moderate matrix surface area. For the operations summarized in Table 1, the percentage improvement in bottoms upgrading ranged from 14 to 32%.
The catalyst matrix plays an important role in improving product selectivities because it is capable of cracking large molecules which cannot readily diffuse into zeolite pores. The fragments which result from matrix cracking can be small enough to enter the zeolite. Both the amount of matrix surface area in a catalyst and the composition of the matrix contribute to its activity for cracking. Although there are some exceptions, in general the amount and strength of acid sites on a silica-alumina matrix, which are responsible for its cracking activity, are associated with its alumina content. The matrix surface area and alumina content of commercial catalysts tend to vary in the same direction as shown in Figure 2.
The effect of active matrix on upgrading bottoms to LCO has been demonstrated in the laboratory as well as in the aforementioned commercial results. Figure 3 shows the bottoms and LCO yields at 70 % conversion as a function of matrix surface area from MAT tests. The laboratory results confirm the observed commercial data which shows that increasing the matrix surface area while holding all other conditions constant, will improve product yield by upgrading bottoms directly to light cycle oil.