Effects of Rare Earth Oxides in FCC Catalysts
Anyone involved with zeolitic fluidized catalytic cracking catalysts quickly becomes aware of the importance associated with the catalyst's rare earth content . . . and often just as quickly confused. Some commercial cracking catalysts have a high rare earth content, while other catalysts contain low levels or even zero rare earth content. Which variety of catalyst is better? . . . Why is it sometimes advantageous to have catalysts containing rare earth and at other times undersirable? . . . and just what are rare earths anyway?
What are Rare Earths?
Rare earth is a generic name used for the 14 metallic elements of the Lanthanide series which contain the atomic numbers from 57 through 71 plus Scandium (At. #21) and Yttrium (At. #39). These elements occupy a unique place in the periodic chart. They are the first elements where the increasing atomic number results in the filling of the inner electron shell after an outer shell has been filled, causing a high similarity in chemical properties. This similarity complicates the separation of one rare earth from another. They are, therefore, often supplied as a mixture of oxides as extracted from ores such as bastnaesite or monazite.
The typical rare earth consists of 46% Cerium Oxide, 20% Lanthanum Oxide, 15% Neodymium Oxide, as well as other oxides of the series. Processors also supply a form of rare earth in which the greater part of the Cerium has been removed. This mixture, known as "Lanthanum rich", contains 6-10% Ce2O3, up to 80% La2O3, 15% Nd2O3 and other oxides. Extensive laboratory tests indicate minimum difference in catalytic performance, and FCC catalysts are accordingly treated with either of the two mixtures.
Why Rare Earths?
. . . Hydrothermal Stability
Lack of hydrothermal stability is the primary reason for adding rare earth oxides to catalytic cracking catalysts. Upon exposure to the high temperatures observed in commercial FCC regenerators the original X-type zeolite cracking catalysts had a tendency to sinter and become amorphous in nature, resulting in a loss of catalyst activity. Addition of rare earth oxides to the cracking catalysts improved their hydrothermal stability; providing stable operations at regenerator temperatures as high as 1300° F.
The subsequent development of Y-type zeolites improved the catalyst's inherent hydrothermal stability. In fact, these newer FCC catalysts without rare earth could withstand temperatures comparable to their earlier counterparts with rare earth. These Y-type zeolites with rare earth remain stable at temperatures as high as 1400° F. Figure 1 illustrates differences in catalyst hydrothermal stability with and without rare earth.
Higher Gasoline Yield and Reduced Gas Make
Rare earth oxides are added to zeolite cracking catalysts through a process known as ion exchange. During this process a portion of the acidic protons and sodium located within the zeolite crystal are exchanged with rare earth ions.
Because rare earth inhibits the dealumination of a zeolite, a higher concentration of acid sites will be found in a rare earth exchanged catalyst. This improves both the activity and the hydrothermal stability of the catalyst. On average, these sites are weaker and in closer proximity to each other than those found in a more highly dealuminated catalyst characterized by lower unit cell size measurements. (See CATALYST REPORT #762 for a definition of unit cell size).
As a result of the greater number of active sites, both the primary cracking and primary hydrogen transfer reactions that occur within the zeolite are enhanced. Primary cracking reactions involve the initial scission of the carbon-carbon bond to form higher valued liquid products such as gasoline. Primary hydrogen transfer reactions are those that occur between cracked products to terminate the cracking reactions in the gasoline range, thus, reducing the overcracking of gasoline to C3's and C4's. The hydrogen transfer reactions are greatly increased with the addition of rare earth to the zeolite.
Thus, rare earth in catalytic cracking catalysts enhances their gasoline yield. Figure 2 is a plot of gasoline yield at varying conversion levels for two standard cracking catalysts with different levels of rare earth. This data suggests a strong correlation between rare earth content and gasoline yield.
Lower Octane and Reduced Cetane Index
The addition of rare earth into the zeolite inhibits the degree of unit cell size shrinkage during equilibration in the regenerator. Steam in the FCC regenerator removes active acidic alumina from the zeolite. Rare earth inhibits the extraction of aluminum from the zeolite's structure (dealumination) which in turn increases the equilibrium unit cell size for FCC catalysts. Since reducing the equilibrium unit cell size of an FCC catalyst has the effect of improving octane, adding rare earth decreases the octane. Further detailed discussions of the effects of reducing a catalyst's unit all size can be found in Catalyst Report Tl-762.
In addition to the above mentioned reactions, the rate of secondary hydrogen transfer reactions is also increased by rare earth. For a rare earth exchanged cracking catalyst the hydrogen transfer reaction of interest is as follows:
Naphthenes(LCO) + Olefins(Gasoline) -> Aromatics(LCO) + Paraffins(Gasoline)
Thus hydrogen transfer reduces the amount of olefins found in the product. These reactions also influence the molecular weight distribution of the product by terminating carbonium ions before they crack to shorter chain fragments. As hydrogen transfer reactions increase relative to cracking reactions, olefin yield, light gas yield and octanes decrease, while gasoline yield increases.
Because rare earth oxides promote hydrogen transfer, the yield of C3 and C4 olefins in the LPG fraction will be lower. (See Figure 3) The resulting reduction in the total LPG yield results in a reduction in the wet gas yield. This reduction in wet gas can have a major effect on plant operations, as compressor capacity is often the limiting factor for FCC unit throughput.
A refiner's own product requirements determine whether a rare earth or non rare earth catalyst is used. If gasoline is desired, a rare earth cracking catalyst should be used; if higher gasoline octane is required, a minimum rare earth catalyst or partial rare earth catalyst should be the catalyst choice. For a catalyst containing reduced levels or no rare earth, the use of an Ultrastable (USY) Zeolite Catalyst is recommended for improved hydrothermal stability.