The Impact of the FCC Catalyst Matrix on Bottoms Upgrading when Processing Heavy Feedstocks
Alan F. Sweezey
BASF Corporation, Houston, TX
When processing relatively heavy FCC feedstocks, a primary objective is to maximize the conversion of the heavy oil to lighter, more valuable products. The FCC catalyst characteristics can play a key role in the ultimate results obtained from each unit. Modern FCC catalysts are primarily composed of crystalline silica alumina zeolites in an inorganic oxide type matrix. The zeolite controls gas oil cracking, while the matrix provides a mechanically stable particle and can contain active sites for cracking heavier hydrocarbons. This Catalyst Report will examine the importance of the catalyst matrix on the bottoms upgrading and conversion obtained from the FCC.
Between the Zeolite and the Matrix
A catalyst deactivation curve (Figure 1) is used to help assess FCC catalyst stability. In this figure, two distinct rates of deactivation are evident. The first four hours show a significant loss of catalyst conversion for both the Rare Earth Exchanged Y Zeolites (REY) and Ultrastable Y Zeolites (USY) type catalysts. This is mainly because of rapid zeolite deactivation that accompanies equilibration of the unit cell size. After this initial deactivation, the rate of conversion loss decreases. Both types of catalysts continue to lose activity and surface area during this period of slower deactivation.
In a commercial FCC unit, catalyst is replaced at a typical rate of 1-5 % per day. This means that the equilibrium catalyst possesses an age distribution. For example, at a 2 % make up rate, about half of the catalyst inventory is younger than 40 days old. There is also a significant catalyst fraction that is over 100 days old. Because of its faster initial deactivation rate, the zeolite activity comes mostly from the young catalyst fraction. Because of the higher activity per unit of surface area for the zeolite compared to the matrix, most of the catalyst conversion also comes from the youngest catalyst fraction.
This principle is illustrated in Figure 2, which shows a catalyst activity distribution for an FCCU. As can be seen from the figure, over 50 % of the total activity in the unit is supplied by catalyst that is 20 days old or less. The 20-40 days old catalyst contributes only 16 % of the unit activity.
Therefore we can think of two different catalyst types within the unit at a given time. The younger particles supply mostly gas oil conversion activity from their zeolite component. But the majority of the catalyst inventory, because of its age distribution, has little zeolite activity. However, it does possess a significant amount of matrix activity (which deactivates more slowly). This older part of the inventory is responsible for providing the activity to upgrade the heavy portion of the feedstock.
Differences Between Matrix
After defining the different roles filled by the zeolite and matrix parts of the FCC catalyst, it is important to understand that catalyst matrices possess different properties depending on their manufacture. BASF's in-situ process makes a catalyst with a matrix that is active and very stable. Figure 3 illustrates the excellent activity retention of the in-situ matrix at very high deactivation temperatures. A typical commercial alumina (alumina B) produced by the incorporation process is shown for comparison. Note that although the initial activity of alumina B was higher than the in-situ matrix, alumina B deactivated much more quickly. These results demonstrate that there are significant differences in the deactivation rates of different matrix types.
In general, the activity of FCC catalyst can be related to its surface area. This conversion relationship can also be extended to the catalyst matrix. To illustrate, several FCC catalysts were severely steam deactivated. Following this treatment, the zeolite surface area is so small that most of the catalytic activity is supplied by the matrix. Figure 4 shows that following this treatment, the catalyst conversion correlates with matrix surface area (there is little zeolite area remaining). This suggests that in a commercial unit, a more stable matrix will supply more activity for upgrading bottoms.
Bottoms Upgrading Assessment
for Commercial Catalysts
The matrix surface area retention of commercial catalysts containing both in situ and incorporated matrix was determined at multiple deactivation temperatures. The results, shown in Figure 5, confirm a significantly higher matrix surface area retention for the in-situ catalysts at all steaming temperatures. Because of the matrix role in cracking heavy hydrocarbons, these results imply a higher level of bottoms upgrading for the commercial in situ catalysts.
To illustrate how the higher matrix stability of the in situ catalyst affects the activity of an equilibrium catalyst, the four commercial catalysts were severely deactivated as in Figure 4. The results are summarized in Table1. Consistent with the previous results, both in-situ catalysts retained significantly more of their original matrix surface area than did the incorporated catalysts. For all catalysts, the rate of zeolite surface area loss was greater than the rate of matrix surface area loss. Therefore, the in situ catalysts should provide a higher level of bottoms upgrading than the incorporated catalysts. Alternatively, a lower catalyst addition rate can be used to maintain the same upgrading level.
What this means to the FCCU in terms of conversion potential is also evident from the table. Similar to the surface area relation, both in-situ catalysts exhibit significantly higher conversions than do the incorporated catalysts. In addition to the increased conversion, the LCO/slurry ratio of the unconverted material is also much higher for the in-situ catalysts.
For the refiner interested in upgrading heavy oils in the FCCU, this means that the in-situ type catalysts will provide more bottoms upgrading activity, particularly from the large part of the catalyst inventory that can be classified as "old". This extra activity provided by a stable matrix can be utilized by the refiner in many ways. For example, the additional catalyst activity could allow the FCC to obtain the same conversion at a lower reactor temperature level. The lower reactor temperature would in turn tend to decrease the light gas production from the unit.
To the refiner that processes heavier FCC feedstocks and wants to maximize conversion of this material, catalysts with an in-situ type matrix provide a greater potential to upgrade heavy oil and minimize slurry production. In addition, these catalysts can provide a higher overall activity to the unit, which can result in a higher conversion level and/or a lower required operating severity for the same conversion.