December 2022

Special Focus: Catalysts

Development/industrial application of FCC catalyst for boosting high-octane gasoline production

Globally, crude oil is trending toward being heavier; the proportion of heavy crude oil reserve is expected to be 50% of the entire recoverable oil reserve moving forward from 2020.

Sha, Y., Han, L., Wang, P., Research Institute of Petroleum Processing (RIPP)

Globally, crude oil is trending toward being heavier; the proportion of heavy crude oil reserve is expected to be 50% of the entire recoverable oil reserve moving forward from 2020 (FIG. 1). The global oil refining business is facing extreme pressures not only due to the heavier and inferior raw materials, but also from the mounting demands for environmentally friendly refining processes and light, clean petrochemical products. This makes the transformation of heavy oil the key factor in the productivity and economic benefits of refineries.

FIG. 1. The increasing trend of heavy oil from 2009–2030.

The pro-hydrogenation of raw materials for catalytic cracking is one of the most well-known techniques to increase operating efficiency and ameliorate the quality of both raw materials and petrochemical products. Nevertheless, the proportion of polycyclic naphthenes and naphthenic aromatics could be enhanced dramatically (e.g., the amount of cyclic naphthenes could account for more than 50%). Moreover, it is well-known that both the increase in heavy oil and the increase of boiling range of distillated oil cause a pronounced growth of the ratio of polycyclic naphthenes and naphthenic aromatics in the raw materials.

For these reasons, one of the most consequential problems in refining techniques is how to make the efficient and rational transformation of polycyclic naphthenes—including the control of the hydrogen transfer (HT) reaction and the promotion of the ring-opening reaction—so that more valuable light petrochemical products can be obtained.

Unfortunately, conventional fluid catalytic cracking (FCC) catalysts show great potential in the HT reaction, yet their abilities in the ring-opening reaction are still relatively low. This accounts for low yield, low octane number of gasolines and the elevation of coke. Therefore, it is imperative to find a strategy to optimize the distribution of catalytic cracking production and enhance the octane number of gasolines.

This article details a new FCC catalyst to concurrently increase the yield of gasoline and decrease the formation of coke. Based on the perceptions of a reaction mechanism, the study also discusses how the designs of the new catalyst developed to meet the aforementioned requirements. Additionally, an industrial application case is introduced.

Strategic design of an efficient catalyst

The most prevalent FCC catalyst is usually comprised of two major components (zeolite and matrix), which lead to two different routes to improve the efficiency of the FCC catalyst: one is to create new zeolite materials, the other is to develop a new matrix.

One of the most widely used zeolite materials in FCC is Y zeolite due to its unique pore structure, high stability and proper acidity. However, the size of polycyclic naphthene molecules is slightly larger than the pore window size of Y zeolite, which impedes the diffusion of those molecules into the pores, thus promoting the secondary reaction. The authors propose that the rational way to augment its efficiency is to create Y zeolite with a small crystal size. Compared to the large-sized Y zeolite, the smaller has a much larger external specific surface area, which increases active sites on the external surface, thus improving the catalytic cracking conversion of heavy oil or residue. Meanwhile, smaller Y zeolite has a better dispersion of acid centers, which accordingly boosts the availability of acid centers. Moreover, the reduction of the crystal size truncates the channel, decreasing the odds of collision between different molecules, which diminishes the yield of cokes caused by the bimolecular reaction (FIG. 2).

FIG. 2. Simplified explanation of difference between different-sized zeolite materials.

Notwithstanding, the hydrothermal stability of the Y zeolite could be marred by decreasing its crystal size. Accordingly, for the development and application of zeolite with small crystal size, it is crucial to improve its hydrothermal stability. With rigorous experiments and theoretical guidance, the authors’ company managed to create a new type of Y zeolite. The size of this new Y zeolite is only ~500 nm, which is about half of the regular NaY. As shown in FIGS. 3A–3D, the stability of the new Y zeolite (marked as SCY) is much stronger than the commercial small-sized Y zeolite while similar to the regular NaY zeolite. It is well-known that the synthesized zeolite materials entail a strong resistance to the steam during the application in a real refinery. Normally, the structure of regular zeolite can be dramatically destructed by the steam at high temperature. To enhance the hydrothermal stability of the authors’ company’s SCY zeolite materials, a gas-phase hyperstabilization method was conducted. As shown in FIG. 3D, although the relative crystallinity of the SCY zeolite is almost identical to the commercial NaY zeolite, the crystallinity retention of SCY is significantly higher than its counterpart.

FIG. 3. TEM image of (A) commercial NaY zeolite, (B) SCY zeolite, (C) crystalline structure collapse temperature of different Y zeolites and (D) relative crystallinity and crystallinity retention of SCY and NaY.

Strategy for the new matrix

In addition to zeolites, matrix should also be considered as another important component in terms of alteration of catalyst activity. The poor accessibility of heavy oil molecules to the active centers of zeolites significantly impedes their diffusion in the zeolite channels. Therefore, the pre-activation of heavy oil molecules on a specific matrix is imperative, requiring the development of a new matrix with proper size. One of the underlying challenges is to balance the matrix’s pre-activated ability and the selectivity of coke formation. Therefore, to curb the formation of coke, the Lewis acid in the matrix must be further reduced and weakened. Compared to a regular matrix, the newly developed matrix, marked as LOM, shows a unique fabric-like morphology with 8-nm mesopores, which is higher than the pore diameter of the regular matrix (FIG. 4). The larger pore diameter is not only beneficial to the diffusion of large molecules, but decreases the possibility of bimolecular reaction and (consequently) the yield of coke. Further experiments confirm that the amount of whole Lewis acid as well as the amount of strong Lewis acid in the novel matrix decreases dramatically, contributing to less coke formation (TABLE 1).

FIG. 4. Pore size distribution curve of (A) commercial Al2O3 and (B) LOM. Insets are the TEM images of commercial Al2O3 and LOM.

Industrial application of the new catalysta

Based on the aforementioned experimental studies, the new catalysta, combined with SCY zeolite and LOM matrix, was successfully produced. It should be highlighted that this catalyst was used in an industrial FCCU from September 15, 2019, to improve the conversion and product distribution of heavy oil and boost the economic performance. The FCCU mentioned is owned by Qilu Petrochemical Co. and the authors’ company, and its designed processing capability is ~800,000 tpy of crude oils. The high and low side-by-side towers are adopted as the reaction-regeneration system. The amount of catalyst added per day is around 2.5 t, which is equivalent to the former catalyst. Considering the standard conditions of changing catalyst and the common cause of catalyst loss, the storage of the new catalysta in the unit reached 100% by March 9, 2021. The catalyst change was performed steadily without any aberrant loss of catalyst. Two representative times were chosen to reflect the potential of the new catalyst. The summary calibration (marked as SC) was conducted from March 9 to March 10, 2021, using the new catalyst, and the blank calibration (marked as BC) was from September 11 to September 13, 2019, when the previous catalystb was used in the same refinery.

Properties of raw oil

The properties of raw materials exert notable influence on catalytic performance, so it is imperative to bring up the properties of raw materials during the calibration. The properties of raw materials during BC and SC are shown in TABLE 2. While the properties from both calibrations are similar, some disparities exist. The carbon residue in SC is ~2.96%, which is 0.96% higher than that in BC. It is supposed that the higher amount of carbon residue would cause an increased formation of coke. Therefore, compared to the raw materials used in BC, those used in SC have an adverse effect on the catalytic activity of the new catalysta.

Basic operating conditions

The basic operating conditions during blank calibration and summary calibration are shown in TABLE 3. Some negligible changes exist between the two calibrations. It should also be noted that the residue ratios in both calibrations are high.

Properties of equilibrium catalyst (E-Cat)

The E-Cat’s properties are shown in TABLE 4, including the specific surface area, the microporous specific surface area, the total pore volume and the volume of micropores. The calibration is discernibly higher than the one used in the blank calibration. The larger specific surface area and pore size are conducive to the interaction between the organic molecules and acid centers on the catalyst and the diffusion of big molecules in catalyst channels, improving the conversion of heavy oil. These results indicate that the new catalysta achieved the designed goal with a greater ability for the conversion of heavy oil. It should be also noted that a stable operation of the unit and a normal fluidization of the catalyst were presented, which means the authors’ company’s catalyst could meet the operating criteria.

Distribution of FCC products and their properties

To present the potential of the new catalyst, the conversion, selectivity, product distribution and properties are provided, as well. As shown in TABLE 5, with the same operating conditions, the yield of gasoline in the summary calibration is 45.9% (1.03% higher than the one in blank calibration) while the yield of coke is reduced by 0.3%—the yield of liquid products was also increased by 1.87%. According to the processing capability and the average prices of the raw materials and products, the cost of the raw materials could be reduced by more than $2 MM/yr while the profit increment from the gasoline could reach $1.8 MM/yr.

Meanwhile, the reduction in coke formation also results in a decrease of CO2 emissions of ~10,000 tpy. The new catalyst does ameliorate the transformation of raw materials and the distribution of products, which can be found clearly in terms of the variation of the yields of gasoline and coke.

It should be noted that the product distribution could vary depending on the extent of reactions. Therefore, a more rational reflection of the product distribution should be selectivity. As shown in TABLE 5, compared to the previous catalystb, the new catalysta shows a better performance in product distribution with a 0.012% increase in the selectivity of gasoline and a 0.005% decrease of coke. The main properties of stabilized gasolines during the calibration are also shown in TABLE 5. The volume fraction of aromatic compounds is decreased from 25.0% to 23.2%, while the volume fraction of alkenes is increased by 4.5%. The amount of benzene is barely changed. The RON of stabilized gasolines is also increased by 0.4%.

All other properties meet the basic criteria, as well. The properties of slurry are also provided in TABLE 5. The density of slurry in the summary calibration is higher. The weight fraction of alkanes is decreased to 7.8% while that of aromatic hydrocarbons is enhanced to 11.9%; the ratio of both gum and asphalt rises, as well. The aforementioned results indicate an increase in the reaction depth, which confirms the ability of the new catalyst to improve catalytic activity.

Distribution of FCC products and their properties

To illustrate the potential of the new catalyst, monthly statistical data from 2019 to 2021 has been provided here, including the residue ratio in raw materials and the yields of gasoline and coke. As shown in FIG. 5, although the residue ratio fluctuates between 40% and 90%, the yields of gasoline and coke remain quite stable. The average yield of gasoline is ~45.5% and the yield of coke is only ~5.8%. With the high residue ratio in the raw materials, the cost of raw materials for the FCC process would be reduced by almost $1,400/yr.

FIG. 5. Monthly statistical data of residue ratio (red), gasoline yield (green) and coke yield (black).

Takeaways

Because FCC raw materials are trending heavier, the need for a more efficient catalyst for the catalytic cracking of polycyclic naphthenes is increasing rapidly. With an improved design of the zeolite material and matrix, the new catalysta  shows promise for industrial applications. The yield of gasoline could reach 55.3% while the yield of coke is only ~5.8%, assuming an average residue ratio of 61.1% in the raw materials. HP

NOTES

a SINOPEC’S ROC-1
b SINOPEC’S RICC-1

The Authors

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