Enhancing bottoms cracking and process flexibility
Catalyst designed with advanced zeolite stabilisation technology provides selective conversion of heavy FCC feed molecules
YEE-YOUNG CHER, ROSANN SCHILLER and JEFF KOEBEL
Grace Catalysts Technologies
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Refiners require FCC catalyst technology that delivers the right selectivity at the right time. In a world where fuel demand is satisfied through a careful balance of free trade, weather events or refinery upsets could trigger price volatility in product markets. The ability to respond quickly to capture short-term market opportunities is critical. Amid declining gasoline demand in mature regions, refiners need to enhance distillate production. Grace’s premium bottoms cracking family, the Midas catalyst series, can be used to enhance FCC process flexibility and capture incremental profit as opportunity arises. These catalysts crack deep into the bottom of the barrel, enhancing total distillate and liquid yield, and have been demonstrated in over 120 refineries that vary broadly in feed composition and operating modes. The flexibility that the catalysts provide, used neat or as a component in a Genesis catalyst system, can enhance the yield value by $0.40-1.00/bbl of FCC feed.1
Midas is a moderate zeolite to matrix ratio FCC catalyst that has been successfully applied in half of North America’s FCC unit capacity as well as refineries in other parts of the world. Its success is driven by the fact that it effectively cracks all feed types: heavy resids, severely hydrotreated light feeds, and shale oil-derived feed streams, via the three-step bottoms cracking mechanism discovered by Zhao.2 The catalyst design minimises the thermal and catalytic factors that result in coke formation. The result is deep bottoms conversion, regardless of the starting feedstock.
Resid streams present the greatest challenge in terms of deep bottoms conversion. The dynamic molecular dimensions of paraffins and aromatic species vary, based on carbon number and molecular configuration. Paraffins species present in the 700-1000°F boiling point fraction of FCC feed are typically in the nC14 to nC34 range for normal paraffins. The dynamic molecular size of these compounds is 12-20 angstroms (Å). The heavy resid fraction also contains an abundance of aromatic molecules (C14 to C60) in the 700-1000°F boiling range. The range of molecular size for aromatics is 12-25 Å. Even aromatic carbon molecules up to 60 carbon number are still less than 30Å in molecular size.
Porphyrins are organic, cyclic macromolecules that consist of a ring of nine or more atoms. Porphyrins are aromatic species often present in resid fractions and characterised by a central gap that can bond to a metal atom, such as nickel, vanadium, or iron. If a porphyrin is complexed with vanadium, it is termed a vanadyl porphyrin. The size of these metallic complexes also varies with carbon number, but is in the same dimensional range as typical resid hydrocarbons: 10-30Å.2
The relatively large molecules at the bottom of the barrel that need to be converted must first be cracked by the catalysts’ matrix acidity. With molecular sizes of 10-30 Å, the hydrocarbons are too large to fit into the zeolite pores, which are typically below 7.5 Å. It is important that the catalyst have the proper pore size distribution to enable large feed molecules to enter, crack into lighter products, and diffuse out before being over-cracked to coke and gas. For free diffusion of resid molecules (>1000°F) to occur, the catalyst pore diameter needs to be 10-20x the size of the molecule, or 100-600 Å.2 The desired pore volume should be in the large mesopore region 100-600 Å. The benefit of mesoporosity for bottoms cracking is well understood.5 However, not all the measured pore volume is created equal. Catalysts with similar total pore volume measurements can vary widely in pore size distribution. Midas is designed to have high mesoporosity in the 100-600 Å range, typically twice as high as competitive offerings (see Table 1). Optimal porosity is required for effective kinetic conversion of bottoms. Midas catalysts crack deeper into the bottoms.
Commercial examples of Midas’s high mesoporosity, as measured by Hg porosimetry of Ecat, are shown in Figure 1. Note that Hg intrusion measures the porosity greater than 36 Å, therefore the result specifies the porosity associated with the catalyst matrix only; N2 adsorption or desorption must be used to capture zeolite porosity. Grace’s in-house manufacturing and quality monitoring of the specialty alumina used in Midas provides control over the resulting porosity. It is generally accepted that micropores (<100 Å diameter), though effective for cracking, lead to poor coke and gas selectivity as a result of poor diffusivity and over-cracking. Some competitive benchmarks with high surface area and activity are also high in matrix microporosity, resulting in wet gas compressor limitations that suppress feed rate and ultimately profitability. In contrast, Midas catalyst has the lowest amount of small pores and the highest amount of large mesopores. Optimal porosity guarantees gas selectivity and coke-selective bottoms conversion. High pore volume also serves to enhance the fluidisation characteristics. Several units have observed substantial improvement in the Ecat fluidisation factor following a reformulation to Midas or Genesis.1
The proprietary matrix in Midas can withstand the most severe applications, particularly those challenged by high levels of contaminant iron and calcium. High alumina content in FCC catalyst is known to reduce the degradation of the catalyst surface due to iron and/or calcium poisoning.4 Optimum distribution of mesoporosity also plays a role in maintaining performance, because diffusion to the active sites remains unhindered despite the high contaminant metals. Midas has been successful in maintaining bottoms conversion in units with some of the highest levels of contaminant iron on Ecat in the industry.
A good bottoms cracking catalyst requires high matrix surface area (MSA). However the activity of a high matrix catalyst needs to be balanced with an appropriate level of zeolite without compromising attrition characteristics. Additionally, an appropriate rare earth exchange level on the zeolite is critical to ensure optimal coke selectivity. An optimum exists in the relationship between zeolite unit cell size (UCS) and coke selectivity.5 Too often, high matrix catalysts also have a high UCS, meaning they are over-exchanged with rare earth. Low zeolite input formulations with high rare earth exchange (albeit low total rare earth on catalyst) will retain a higher percentage of the fresh zeolite surface area in a severe regenerator, but the penalty for this over-exchanged zeolite is poor coke selectivity. The rare earth exchange in Midas catalysts is optimised to result in a formulation that lies in the so-called ‘sweet’ spot or between 24.28-24.32 Å for Ecat UCS to deliver the best coke selectivity. The right distribution of mesoporosity coupled with optimal activity and UCS gives the catalyst the coke selectivity edge in commercial cracking.
Midas’s design features have led to high performance in many commercial examples. The first example comes from Refinery A that has an FCC unit processing a mix of VGO and light resid. The feed is moderately high in metals with good cracking characteristics. The base catalyst was a competitive high MSA catalyst with high rare earth exchange. The unit suffered from a dry gas constraint, especially in the summer. When the refiner switched to Midas, a 5-10% drop in total dry gas yield at equivalent riser temperature and metals loadings was observed; this allowed the refiner to maintain maximum feed rate throughout summer. The better coke selectivity also manifested itself in lower bottoms yield by 15%.
At Refinery B, the FCC unit was processing light resid using a high matrix, high UCS catalyst. While rare earth levels on the zeolite were high, the catalyst itself had low total rare earth because zeolite input was very low. In spite of the high rare earth exchange and resulting UCS, the unit was constrained on total wet gas and bottoms yield. There wasn’t enough zeolite present to complete the second stage of the bottoms cracking mechanism.2 Zeolite is critical to reduce the size of the hydrocarbon via dealkylation. Insufficient zeolites cause coke precursors to condense on the catalyst surface and become coke before they can be converted into transportation fuels. Poor coke selectivity translates into subpar bottoms conversion.
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