Maximising FCC distillate production
A novel catalyst manufacturing process forms matrix material and zeolite in a single step. Trials indicate high matrix stability and low sodium content
Joe McLean, BASF Catalysts
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Increased demand on refiners to process heavier crudes and maximise diesel yields (see Figure 1)1 means FCC units (FCCUs) are continually pushed to their operating limits. Higher diesel yields are often achieved through the use of selective, bottoms upgrading FCC catalysts that feature active matrices. However, these conventional bottoms upgrading catalysts also increase the yield of undesirable products such as dry gas and coke. This often pushes the FCCU up against the limits of performance in terms of gas compression and metallurgy, preventing the refiner from taking full advantage of the potential for increased diesel yields.
Maximising distillate: fundamentals
The refining industry would like to convert feed/bottoms in an FCCU to light cycle oil (LCO) without further cracking to gasoline. Adjustments to the operating conditions that can be applied to an FCCU include reducing riser temperatures and/or catalyst activity by lowering the catalyst make-up rate (see Figures 2 and 3). However, both of these options lead to an increase in bottoms yield greater than the increase in yield of LCO. This clearly is not desirable.
Another way to increase the production of LCO is to recycle slurry/bottoms. Second-pass yields are typically less selective and lead to higher coke and gas production. Use of recycle in an FCCU constrained by air blower capacity can result in a reduction in feed rate, which can reduce the total production of LCO. No adjustments to the FCCU, or combination of adjustments, will alone result in increased output of LCO without at the same time increasing bottoms production. Only an improvement in catalytic performance offers a pathway to increased LCO production while maintaining bottoms conversion.
With BASF’s Novel Matrix Material, LCO yield can be increased by controlling the depth of cracking. The conventional catalyst approach to increasing LCO yield has been to lower the Z/M ratio, typically by both increasing the matrix surface area (MSA) and lowering the zeolite surface area (ZSA).
BASF has introduced a manufacturing process for an advanced matrix material. This is based upon a novel technology platform that differs from more conventional matrix materials available to the industry. Figure 5 compares the hydrothermal stability of the new matrix material with a variety of commercially available matrix materials. It shows that the novel manufacturing process has resulted in a matrix material that demonstrates improved hydrothermal stability.
Most conventional matrix materials, such as alumina, predominantly have Lewis acid sites, measured by pyridine adsorption. The novel manufacturing process generates not only Lewis acid sites, but also a small but significant fraction of Brønsted acid sites (see Table 1). The role of Brønsted acid sites in zeolitic cracking is well known. The Brønsted sites on the matrix material, although weaker than zeolitic sites, are believed to act synergistically with the Lewis sites to enhance catalytic properties.
However, a lower Z/M ratio results in accentuated matrix â€¨cracking with poor coke and gas selectivities. Figures 4a and 4b show a comparison of LCO and coke selectivities for two conventional catalysts at two different Z/M ratios. As preceding literature indicates, lowering Z/M does increase LCO yield, but at the expense of higher coke production.
This demonstrates a need for a fundamentally different catalytic approach to increasing LCO yield. The need is to have a selective matrix that would increase the LCO yield without the accompanying coke yield.
New technology platform
Development of the novel matrix material was extended to include both the crystallisation of Y zeolite and the formation of the matrix in a single step. The manufacturing process forms both the matrix material and zeolite in a single step, and it brings them into very intimate contact, with an ultra-low sodium content. The hexagonal crystallites of Y zeolite, smaller than 1 µm, are in the immediate vicinity of, if not in intimate contact with, the amorphous matrix materials.
Catalytic properties of the different matrix materials were compared by preparing physical blends of matrix materials with separate particle US-Y zeolite, which had no matrix. Both were separately deactivated at 1500°F (815°C), four hours and 100% steam. US-Y zeolite had 2.6 wt% rare earth oxides (REO). The blend ratio was adjusted to maintain a constant Z/M ratio. The blends were evaluated for cracking performance in the advanced cracking evaluation (ACE) unit at 970°F (520°C), with hydrotreated feed â€¨containing 3.88 wt% Conradson carbon residue. LCO selectivity is defined as the ratio of LCO to the sum of LCO and heavy cycle oil (HCO). Three different matrix materials used in other FCC catalysts were compared to the new matrix material.
Figure 6a shows the plot of coke versus conversion for the four matrix materials tested. The competitive matrix materials had very similar coke yields. In contrast, the proprietary matrix stood out with lower coke yield. Figure 6b shows a plot of LCO selectivity versus coke yield for the four blends. The other three matrix materials had similar LCO yields, whereas the proprietary matrix showed higher LCO selectivity.
While other catalyst technologies can incorporate zeolite and matrix materials in the same catalyst particle, they do not have the capability to bring them together in such intimate contact. It is this synergy created by the close proximity of zeolite and matrix that leads to rapid transfer between reactant and feed molecules from zeolitic acid sites to matrix acid sites. The enhanced transfer results in the coke precursors produced by matrix cracking being stabilised by zeolite, leading to higher LCO production with lower coke.
The benefit of zeolite and matrix synergy was evaluated in a circulating riser unit (CRU) with a mildly hydrotreated gas oil (API gravity 23.2º and Conradson carbon residue content of 0.4 wt%). The base catalyst was a Distributed Matrix Structures (DMS) based, high Z/M catalyst targeted for making gasoline. The base catalyst was blended to a 30% level with either the new BASF catalyst or a commercially available matrix material designed for making LCO. The results are shown in Table 2.
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