Short Contact Time Risers Require High Activity Catalysts

by Michael Hurley

Introduction

Past data, compiled by BASF, have shown that the majority of units incorporating advance riser termination devices (RTD) have changed to using more active catalysts due to the reduced catalyst-to-oil contact time. This reduced contact time has benefited the refiner by reducing dry gas yields; however, many have actually seen an increase in bottoms production and/or a significant reduction in regenerator bed temperature thus requiring increased catalyst activity to re-optimize the unit operation.

This Catalyst Report illustrates the unique ability of insitu-produced BASF catalysts to meet both this catalytic activity requirement of RTD's and the requirement for physical ruggedness.

Discussion

Industry Trends
The table below illustrates an industrial average of fresh catalyst properties before and after implementing advanced riser termination devices.

These data clearly illustrate the industry trend toward higher activity catalysts after incorporating an RTD.

BASF is uniquely positioned to meet the wide spectrum of catalysts required by the petroleum refining industry, including high activity catalyst for units with RTD's. BASF manufactures FCC catalyst using three different processes: the incorporation process, the in-situ process, and a combination of the two called the 'Flextec' process. However, products produced by the in-situ process have unique properties making them the preferred catalyst for units with RTD's.

Comparison of Manufacturing Processes
The move toward higher activity catalysts has made products produced by the BASF in-situ process the choice of many refiners in the industry. The ability to grow zeolite on a prehardened microsphere in the in-situ process results in increased total surface area and attrition resistance when compared to the incorporation manufacturing process. The basic problem with high activity incorporation products is an inability to retain both desirable zeolite and matrix surface area and attrition resistance simultaneously.

Attrition Resistance
Activity, in the form of surface area, is limited in the incorporation process due to the need for suitable binder to complete the structural framework of the catalyst. As more surface area (matrix and zeolite) is incorporated into the catalyst particle, the attrition resistance of the catalyst is weakened. This lower attrition resistance is somewhat offset by use of improved binder technologies; however, such improved binder technologies are still insufficient to produce an incorporated product with attrition characteristics comparable to those of an in-situ product. Figure 1 illustrates the effect of increased surface area on the attrition resistance of the catalyst particle for supplier A and B (incorporation process) and BASF (in-situ process).

The importance of attrition resistance is heightened as units continue to adopt modern technology. Upon revamp, units circulate at higher velocities, include more steam in both the feed zone and transfer lines, and many times increase regenerator dilute phase velocities. All of the above individually and/or combined increases the demand on the catalyst attrition resistance to maintain a physically sound and environmentally safe process.

Matrix Activity
Despite the negative impact that post riser cracking has on dry gas production, evidence from actual operation now suggests that a portion of bottoms cracking prior to implementation of an RTD was accomplished thermally in the reactor dilute area. The elimination of this thermal cracking now requires additional matrix to effectively reduce slurry yield by reaction in the riser. As a consequence, a significant portion of the additional activity required after incorporating an RTD is in the form of additional matrix surface to insure effective bottoms cracking.

This need for additional matrix is readily satisfied by a strength of BASF's in-situ catalysts. The matrix manufacturing technology of the in-situ process produces the most stable matrix in the industry. Figure 2 illustrates retention characteristics of in-situ matrix when compared to matrix manufactured in the incorporation process. Note that while the incorporation process can produce high fresh matrix surface area, as noted in the previous section, attrition resistance suffers by doing so.

The incorporation process must include either additional alumina or treated clays in the catalyst to provide the necessary matrix needed in units with RTD's. Furthermore, these added substances are far less resistant to the harsh mechanical and hydrothermal environment of the FCC process. This harsh environment, in turn, requires the use of even more fresh matrix surface area to equilibrate at the necessary levels to effectively crack the feedstock heavy ends.

Zeolite Activity
Given the ability of the insitu product to retain a higher percentage of fresh matrix in the process environment, it follows that for a given equilibrium catalyst matrix surface area, the insitu product can have more fresh zeolite per catalyst particle when compared to an incorporation manufactured catalyst whose total surface area is limited due to attrition concerns. Since there is general agreement that activity via increased zeolite surface area rather than matrix has distinguishable advantages in product selectivities, the insitu product has this added economic advantage. Figure 3 illustrates the advantage that the insitu process has over the incorporation process for providing a higher zeolite surface for a given matrix surface area.

Rare Earth
To compensate for the inability to provide the necessary surface area to meet the unit activity requirement, those units utilizing incorporation technology are forced to increase rare earth on zeolite. Much of the time, the rare earth is also increased in those units that use an in-situ product after implementing an RTD; however, the increase necessary to accomplish the desired activity is less than that needed for an incorporation catalyst. The increase in rare earth is lower, primarily due to the higher zeolite surface area and more stable matrix of the in-situ product.

In many cases, the product selectivity shifts caused by increased rare earth can have a negative economic impact. In fact, the adoption of RTD's is many times justified economically based not only reducing dry gas but also on increasing severity, producing more C3/C4 olefins and producing higher octane gasoline. If unit operation forces an increase in rare earth beyond reasonable levels on incorporated catalysts, both LPG olefinicity and gasoline octane are adversely affected.

Conclusion

The evidence above illustrates why an FCC catalyst produced by BASF's proprietary insitu manufacturing process is preferred for those units that have installed an RTD and require increased catalyst activity. The increased stability of the in-situ matrix coupled with the ability to provide more zeolite and additional activity with less rare earth can give the refiner a significant operating advantage. These advantages have resulted in BASF currently supplying catalyst to more than 40% of those units that have adopted advanced riser termination devices(Ref. 2).

References

1,2 E.B. Bovo and J.B. Mclean, 'FCC Catalyst Trend's: Responding to the Challenges of the 1990's' NPRA Annual Meeting paper AM-95-64.