Fluid catalytic cracking (FCC) continues to be a significant process for the conversion of low value heavy oils to higher value hydrocarbon liquids and, in particular, it is used to meet the supply demands by converting heavier gas oils to gasoline. Vaporised hydrocarbon feeds are mixed with the FCC catalyst, and the fluidised catalyst and hydrocarbon vapours react while flowing upward in a reactor producing gasoline and other high value products. FCC catalysts are separated from the products and regenerated to remove any coke deposits that may have formed during the catalytic cracking. The regenerated and active material is commonly referred to as an equilibrium catalyst (ECAT). The cracking reaction and the catalyst regeneration are conducted at elevated temperatures, often greater than 480°C and 600°C, respectively. Regenerated catalysts may then be reused in the FCC process and thus provide a closed cycle for the catalyst and efficient conversion of heavy oil to high value liquids such as gasoline.
Balancing composition and size to deliver an active catalyst
The FCC catalyst is typically a combination of a crystalline alumino-silicate, most commonly zeolite Y, and kaoline. The Y zeolite (faujasite) has the active acid sites for the catalytic cracking and suitable pore size and network that provide shape selectivity for the catalytic cracking of heavy oils to produce lower boiling hydrocarbons for the gasoline pool. FCC catalysts are characterised by approximately 0.74 nm pore openings and acid sites that – dependent upon the silica to alumina ratio of the faujasite and lower ratios (indicating larger alumina content) – have a larger number of acid sites. The porosity of the material is dependent upon more than simply the faujasite content. While the porosity of the faujasite provides a very high surface area and thus a large number of catalytic sites, the kaolin is used to control the particle size of the FCC catalyst. The particle size distribution, shown in Figure 1, is critical for good performance and in this application the size of the finished catalyst must provide a powder that is small enough to be readily fluidised by the vaporised feed, but the particles must be sufficiently large so that they are easily separated and recovered from the product stream to then be regenerated for reuse. The FCC catalyst is subject to conditions that often promote the formation of smaller particles commonly referred to as fines, which may be subject to attrition and require the addition of fresh FCC catalyst on a regular basis.
Figure 1. Particle size distribution of FCC catalyst (courtesy of Jack Saad using the Micromeritics Saturn DigiSizer).
Using adsorption to explore porosity
The texture of the FCC catalyst particle, porosity and pore size, is engineered to yield a surface that has an abundance of active sites for cracking and a pore network optimised for efficient mass transfer of reactants to the active sites and products away from the active sites. The optimised pore networks contain both micropores (ca. 0.7 nm) from the faujasite and mesopores (2 – 50 nm) resulting from the production via the combination of kaolin and faujasite, or from the catalyst manufacturer steaming the faujasite, to produce ultra-stable Y (USY) used for the FCC catalyst. The mesopore size and connectivity may dramatically impact the rate of reaction and performance of the catalyst. A highly optimised catalyst will often feature a connected mesopore network that consists of uniform and open channels (pores) that allow reactants and products to readily diffuse through the catalyst. FCC catalysts that contain pore networks containing severely constricted or occluded pores may exhibit poor performance and a higher tendency for coke formation and, ultimately, the blocking of pores and reduced access to the active acid sites required for cracking.
Adsorption of nitrogen or argon is typically used to assess the surface area and porosity of an FCC catalyst. The adsorption isotherm can then be used to determine the size and quantity of micropores (< 2 nm), the surface area (via the BET method), the mesopores (2 – 50 nm), and the pore volume of the FCC catalyst. Figure 2 provides example adsorption isotherms for zeolite Y (faujasite), USY (steam treated Y), USY-A (acid leached USY), and USY-H (base treated USY-A). The adsorption isotherm shown in Figure 2 for the parent faujasite (Y) demonstrates an IUPAC Type I classification commonly attributed to a microporous material, and adsorption at low relative pressures is characteristic of the zeolite pore window that serves as an entrance (ca. 0.74 nm) to the larger pore (ca. 1.1 nm). The USY exhibits an isotherm similar to Y but also features hysteresis during desorption and this is an indication of mesoporosity with pores > 2 nm. Additional post-synthesis techniques, acid leaching and base leaching were used to demetallate the USY and further enhance the mesoporosity, resulting in increased hysteresis (Figure 2 USY-A and USY-H).
Figure 2. Nitrogen adsorption isotherms for zeolite Y, USY, acid leached to produce USY-A, base treated to produce USY-H (isotherms obtained using Micromeritics 3Flex).
Assessing surface area using standard methods
The isotherms may then be used to estimate surface area and nature of the porosity using well-established methods (Table 1). The BET1 method is commonly used to calculate the surface area of porous and non-porous materials and using the portion of the isotherm that is characteristic of nitrogen monolayer and multilayer development during adsorption. In addition to surface area, the t-method2 provides complementary information including the matrix area and pore volume of the zeolite. The matrix area is also commonly referred to as the external area and this is the surface area excluding the micropores.
The textural analysis of the Y zeolite and the three versions of USY indicate the effect of the post-synthetic treatments. Steaming of the parent Y, thus producing USY, resulted in a minor loss of surface area with the pore volume remaining unchanged. The demetallation treatments, acid washing (USY-A) and base treating (USY-H), significantly increased both BET surface area and pore volume, as well as reducing the micropore area and volume. For both USY-A and USY-H, the matrix area increased significantly when compared to the Y and USY values. These changes indicate increasing mesoporosity in the FCC catalyst, which may improve overall mass transfer within the catalyst particle. Surface area and pore volume are common properties that can be monitored to provide feedback on the status of the FCC catalyst, which may in turn correlate to changing process conditions such as operating temperature of the FCC unit.
To read the rest of this article, as well as other articles from the September 2020 issue of Hydrocarbon Engineering, click on this link: https://bit.ly/2G3bpxR
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/13102020/precise-catalyst-evaluation/
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