Jan-2013

Enhanced bottoms cracking and process flexibility with Midas FCC catalyst

This premium bottoms cracking FCC catalyst family is formulated with advanced zeolite stabilisation technology plus matrix metals passivation technology, and has been successfully applied to a wide range of feed and feed contaminants

Yee-Young Cher, Rosann Schiller and Jeff Koebel
Grace Catalysts Technologies

Article Summary

Middle distillates, including on-road and off-road diesel, marine fuels and No. 2 heating oil, represent the largest and a growing market share of refined petroleum products. However, the renaissance of North America crude production will have a profound effect on refinery yield slate. Many of the new crudes are lighter and will increase refinery light ends yield. Growing volumes of domestic crude are being processed outside of PADDs II and III despite pipeline congestion.¹ Several refiners have announced projects to source light oil via rail, barge or truck to plants along the US East and West coasts.² Longer term, surpluses are expected in the lighter products (LPG and naphtha) and deficits forecasted for gas oil and diesel.

Overall, the trade balance for distillate streams is tight globally. The closures of large refining complexes in the Atlantic basin, changing fuel specifications, and growing demand for gas oil and distillate in the emerging regions and Europe add to the expected supply imbalance. Amid declining gasoline demand in North America, refiners require an FCC catalyst that enhances distillate production yet selectively minimises light product yield. In a world where fuel demand is satisfied through a careful balance of free trade, a single storm or refinery upset often triggers price volatility in product markets. The ability to respond quickly to capture short-term market opportunities is critical. Grace’s premium bottoms cracking family, the Midas catalyst series, can be used to enhance your FCC process flexibility and capture incremental profit as opportunity arises. Midas cracks deep into the bottom of the barrel, enhancing total distillate and liquid yield, and has been proven in over 100 refineries that vary broadly in feed composition and operating modes. The flexibility that Midas brings to your refinery, 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.³

Midas is a moderate zeolite-to-matrix ratio FCC catalyst that has been successfully applied in half of all the North America FCCU capacity. Its success is driven by the fact that Midas effectively cracks all feed types: heavy resids, severely hydrotreated light feeds and shale oil-derived feed streams, via the three-stepbottoms cracking mechanism discovered by Zhao.4 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 12-25Å. 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.⁴

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 10-30Å, the hydrocarbons are too large to fit into the zeolite pores, which are typically below 7.5Å. 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 100-600Å.4 The desired pore volume should be in the large mesopore region 100-600Å. The benefit of mesoporosity for bottoms cracking is well understood.7 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 100-600Å range, typically twice as high as competitive offerings (Table 1). Optimal porosity is required for effective kinetic conversion of bottoms. Midas catalysts crack deeper into the bottoms than competitive technologies in the market today.

Commercial examples of high Midas mesoporosity, as measured by Hg porosimetry of E-cat, are shown in Figure 1. Note that Hg intrusion measures the porosity greater than 36Å, 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 speciality alumina used in Midas provides control over the resulting porosity. It is generally accepted that micropores (<100Å diameter), although 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 best-in-industry 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 E-cat fluidisation factor following a reformulation to Midas or Genesis.3 Later in this paper, the selectivity advantages of Midas will be highlighted with commercial examples of catalytic performance.

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 Fe and/or Ca poisoning.7 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 Fe on E-cat in the industry.