March 2022

Special Focus: Petrochemical Technology

New practice of diameter-transformed fluidized bed reactor in the petrochemical industry

Motor gasoline standards are becoming increasingly stringent—in particular, the olefins content requirement in motor gasoline is getting lower and lower. Among these standards, China standard motor gasoline VI (B) requires that the olefin content should be < 15="">

Motor gasoline standards are becoming increasingly stringent—in particular, the olefins content requirement in motor gasoline is getting lower and lower. Among these standards, China standard motor gasoline VI (B) requires that the olefin content should be < 15 vol%. However, the olefin in China’s gasoline pool is mainly from fluidized catalytic cracking (FCC) gasoline. Therefore, the production of low-olefin gasoline is a vital issue in the development of FCC technology.

With the rapid development of the olefin industry, a strong demand exists for chemical raw materials like ethylene and propylene. Increasing the production of propylene has become one of the important tasks of an FCCU. Based on research on the catalytic reaction pathway and the mechanisms of petroleum hydrocarbon molecules, the authors’ company proposed the concept of a fluidized bed with a transformed diameter to construct different reaction zones,^1–8 and invented a novel diameter-transformed fluidized bed (DTFB) reactor.^9–11

Based on this novel reactor platform, a series of technologies have been successfully developed: maximizing iso-paraffins in cracked naphtha (MIP),^12,13 cleaner gasoline and more propylene (CGP),^14,15 LCO to gasoline (LTG),^16,17 and integrated technology of hydrotreating FCC gasoil and highly selective catalytic cracking for maximizing liquid yield (IHCC),^18,19 which have achieved large-scale applications. These technologies play an important role in clean fuels production, product structure adjustment and refining technology upgrading that produce good social and economic benefits.

DTFB reactors have been applied to more than 100 industrial catalytic cracking units²⁰ by a patent licensed mode with an annual processing capacity of more than 120 MMtpy. DTFB reactors have been developed into an open engineering technology platform—recently, the catalytic cracking technologies for the production of ultra-low olefin gasoline and for more propylene and marine fuel oil components have been developed on this platform. These new technologies have injected new vitality into the development of catalytic cracking technology.

What is a DTFB?

A DTFB reactor, shown in FIG. 1, is a multi-flow, single-vessel fluidized bed that is organically composed of a transport bed, a fast fluidized bed and a turbulent fluidized bed. It mainly includes the first reaction zone (transport bed), the second reaction zone (turbulent fluidized bed and fast fluidized bed), the outlet zone (transport bed), transition zone, gas-solid fluidized distributor and spent catalyst circulating pipe (not shown). A DTFB reactor can not only achieve a low-temperature (with injection of quench medium or spent catalyst) or high-temperature (with injection of semi-regenerated catalyst or regenerated catalyst) environment in the second reaction zone, but also provides enough time, sufficient space and an abundant amount of catalyst particles. These favorable conditions can promote gasoline olefin molecules to undergo secondary reactions, such as isomerization, selective hydrogen transfer and re-cracking.

Quite different from the conventional riser, a DTFB has the characteristics of a multi-flow type and multi-temperature zone that can meet the different requirements of different chemical pathways of reactants in kinetics and thermodynamics. A DTFB realizes a new riser temperature distribution, a new catalyst concentration distribution and a new oil and gas velocity distribution.

A DTFB’s reaction mode design

From the viewpoint of FCC reaction chemistry, hydride transfer reaction plays a key role as it is not only the elementary reaction of bimolecular cracking reaction, but also the elementary reaction of the hydrogen transfer reaction. In the presence of solid acid catalyst, carbenium R₂⁺ will extract hydride ions from the raw material molecules (e.g., alkanes) to undergo a hydride transfer reaction, which converts itself into product alkanes (R₂H) and makes the raw material molecules form a new R₁⁺. After that, the entire catalytic cracking reaction continues.

Based on the hydride transfer reaction, the bimolecular reaction evolves into an increasingly complex reaction system as the reaction depth increases. The optimization of the reaction depth and the control of the reaction direction are the fundamentals for achieving directional control of product distribution. Among them, hydrogen transfer reaction is divided into two types, as shown in FIG. 2, and the involved chemical bond evolution includes double bond saturation, aromatization, isomerization, disproportionation, etc. Therefore, further deep hydrogen transfer will form coke precursors. The conversion rate is an indicator of the depth of the reaction, and the direction of the reaction is to regulate the type of hydrogen transfer reaction, and then directionally regulate the product distribution and gasoline composition. Based on these studies, the mechanism and method for the selective regulation of the bimolecular reaction and the optimization of the reaction depth by the hydride transfer reaction were discovered.

If the hydrogen transfer Type I is enhanced, gasoline olefins are converted into iso-paraffins and aromatics, which are beneficial to increase gasoline octane number. If hydrogen transfer Type II is enhanced, gasoline olefins are converted into iso-paraffins with different carbon numbers, but the yield of coke precursors is also increased. Also, excessive intensification of hydrogen transfer Type II can easily lead to a substantial increase in the yield of coke. The relationship between conversion (reaction depth) and gasoline composition (reaction direction) is shown in FIG. 3.

Conversion rate has a great influence on the composition and octane number of gasoline and is defined as the mass fraction of coke, cracked gas and gasoline. FIG. 3 shows PONA data of gasoline at different conversion rates. When the conversion rate is < 65%, the gasoline olefin content slowly decreases as the conversion rate increases, the iso-paraffin content slowly increases, while aromatic content remains almost unchanged. When the conversion rate is between 65% and 80%, the olefin content decreases rapidly, and both iso-paraffins and aromatics increase significantly. When the conversion rate further increases, the olefin content continues to decrease but the extent slows down. Therefore, to further reduce gasoline olefins in an FCC process, increasing the conversion rate is the main direction for the adjustment of the reaction mode.

By means of DTFB reaction engineering technology and the structural optimization of molecular sieve catalysts, big data analyses of all factors affecting conversion rate and transfer reaction type are carried out—it is found that the reaction temperature and catalyst activity are the most significant. The reaction mode between catalyst activity and reaction temperature is established and a relationship diagram is shown in FIG. 4.

Based on the reaction mode constructed in FIG. 4, the conversion rate can be adjusted, while the type of hydrogen transfer reaction can be adjusted—then, the gasoline composition can be directionally adjusted. From the selective transfer reaction mode (Mode 2), the reaction temperature and catalyst activity are further optimized to construct a high-severity reaction mode (Mode 3 or mode 5), which will increase the conversion rate and reduce the olefin content of gasoline. Tests have shown that the olefin content of gasoline in Mode 3 can be reduced to less than 15%, and the content of iso-paraffins has increased significantly. Based on the regulation of bimolecular hydrogen transfer reaction, the concept and method of selective hydrogen transfer were developed—that is, hydrogen transfer Type I should be carried out as much as possible to reduce coke formation. This has laid a solid foundation for the development of corresponding processes and catalysts.

However, as the olefin content of gasoline is greatly reduced, the coke yield is greatly increased (FIG. 5), making it difficult to operate smoothly in Mode 3 and presenting challenges to the normal operation of the FCCU. The main reason for the substantial increase in coke yield is that the fused ring aromatics and olefins continue to undergo hydrogen transfer reaction Type II, and the condensation reaction produces coke. Therefore, the contradiction between the deep reduction of olefins and the rapid increase in coke yield is a scientific problem to be resolved.

DTFB to produce ultra-low olefins gasoline: The discovery of hydride ion release agent and its catalytic conversion path

Based on the test result that the selectivity of 2-methyl-2-butene’s hydrogen transfer reaction on different spent catalysts is less than 20%, to further reduce gasoline olefins, it was proposed to find a kind of hydride ion release agent to strengthen the hydride ion transfer reaction. A large number of experimental verifications have shown that tetralin and decalin are potential hydride ion release agents, which can promote 70% of 2-methyl-2-butene to undergo hydrogen transfer to form isopentane. At the same time, tetralin and decalin are converted to naphthalenes, and the coke yield of the reaction decreases instead.

Then, the catalytic reaction test of a mixture of olefin-rich gasoline and tetralin was conducted. As the concentration of tetralin molecules in the olefin-rich gasoline increases, the yields of dry gas and coke show a significant decrease. Meanwhile, the content of olefins gradually decreases—at its lowest, reaching 4%—and the content of paraffins and aromatics increases. This shows that the reaction between olefins and tetralin molecules is mainly hydrogen transfer reaction Type I, which produces paraffins and aromatics, rather than hydrogen transfer reaction Type II, which produces coke precursors. The role of tetralin in the reaction process with olefins and its catalytic conversion path was determined. Only a small part of tetralin undergoes β-scission reaction (FIG. 6), while most of them form aromatic hydrocarbons through hydride ion transfer reaction and proton removal reaction (FIG. 7). In this process, tetralin replaces the polycyclic aromatic hydrocarbons (PAHs) in the original system to release hydride ions, thereby inhibiting the condensation of PAHs to form coke without causing a rapid increase in coke yield. At the same time, it can significantly promote the hydrogen transfer reaction of olefin molecules to generate paraffins.

Development of catalytic cracking technology for ultra-low olefin gasoline

A methodology and mixing ratio of a hydride ion release agent into the reaction system are proposed here. The most typical compounds of hydride ion release agents are tetralins and decalins. FCC light-cycle oil (LCO) contains many naphthalene compounds. After hydrotreating, naphthalene compounds become tetralins and decalins. In other words, these compounds can be derived from hydrotreated FCC LCO.

The hydride ion release agent is similar to a working fluid, which circulates between the catalytic cracking unit and hydrotreating unit to release hydrogen or hydrogenate. The reasonable way to introduce the hydride ion release agent to the fresh heavy oil reaction system is to directly mix the hydrogenated LCO with fresh heavy oil and then introduce the mixture into the riser reactor. This can significantly improve the physical properties and chemical reaction performance of the heavy oil.

The mixing ratio of hydrogenated LCO and fresh heavy oil was systematically studied. It was found that when the mixing ratio of hydrogenated LCO exceeds 10%, the viscosity of the heavy oil can be significantly reduced, which can reduce the dry gas and coke yield in the catalytic cracking process of heavy oil feedstock. In this process, the hydrogenated LCO itself is a high-quality heavy oil emulsifier. As a result, a catalytic cracking technology [ultra-low olefins (ULO)] for the production of ultra-low olefin gasoline was developed. Industrial operating data of ULO technology are shown in TABLE 1.

With the introduction of an external hydride ion release agent, the contradiction between the deep reduction of gasoline olefins and the rapid increase in coke yield has been fundamentally resolved. As shown in TABLE 1, with the introduction of a hydride ion release agent (mixing ratio of 12%) into fresh heavy oil, gasoline olefin content decreased to 8.5%. When the conversion rate increased from 70.6% to 78.8%, the coke yield increased slightly.

Compared with advanced catalytic cracking technology (MIP), the gasoline composition of the ULO technology is quite different (FIG. 8). MIP gasoline has high isoparaffin content and moderate olefin and aromatic content, the result of both the cracking reaction and hydrogen transfer reaction. ULO gasoline has much higher isoparaffin content, with an olefin content less than 10%, and slightly higher aromatics content, the result of strengthening the selective hydrogen transfer reaction.

Producing more propylene and marine fuel oil components

With the progress of the times, the products produced by DTFB can be transformed from conventional gasoline and diesel to light olefins and marine fuel oil, which means a DTFB continues to play an important role in the petrochemical industry. By using three different types of heavy oil, a DTFB produces more propylene and marine fuel oil components. The test results are listed in TABLE 2.

From TABLE 2, with hydrogenated vacuum gasoil (HVGO), vacuum residuum desulfurization (VRDS) or atmospheric resid (AR), a DTFB can directly produce propylene with a yield of > 10% and a marine fuel oil component yield of > 28%. If the operation mode of light gasoline or light diesel recycling is adopted, the propylene yield will be higher. Therefore, a DTFB has achieved another transformation and upgrading.

Advances in processing hydrogenated heavy oil

In recent years, the trend of heavy and inferior crude oil has become prominent, and the price gap between high-sulfur crude oil and low-sulfur crude oil has widened. To improve the utilization efficiency of petroleum resources and increase economic benefits, refiners are favoring the combined processing route of the residual oil hydrogenation unit and the catalytic cracking unit. Compared with straight-run residue oil, the product distribution of catalytic cracking of hydrogenated heavy oil has been significantly improved, and product properties—especially the olefin and sulfur content of gasoline—have been greatly improved.

TABLE 3 lists the overview of three sets of DTFBs for processing hydrogenated heavy oil, including the main properties of raw materials, product distribution and gasoline properties during calibration. For hydrogenated heavy oil, the product advantages of a DTFB reactor are low olefin content in gasoline and high RON. Additionally, with special catalysts and a flexible switch of the reactor operation mode, it can produce more propylene.

Advances in processing paraffin-base heavy oil

With the development of special catalysts and new DTFB engineering technology, the adaptability of a DTFB in processing paraffin-base heavy oil has been greatly improved. For a clean gasoline production program, compared with an early DTFB, the olefin content of gasoline can be reduced from 35% to less than 20%, while RON increases.

Using a petrochemical company that is processing paraffin-base heavy oil as an example, the changes in gasoline properties before revamp (adopting a conventional riser) and after revamp (adopting a DTFB) are shown in FIG. 9. It can be clearly seen that the olefin content of gasoline produced is ~30% and RON is ~90 when the conventional riser is used and the equilibrium catalyst activity is 70 (the unit consumption of fresh catalyst is 1.6 kg/t of heavy oil). After revamp into the DTFB, the olefin content of gasoline can be reduced to < 20%, the olefin content of the gasoline is 23%–25% and RON is 90.5–91.1 after the operation mode adjustment.

At this time, the unit consumption of fresh catalyst is only 0.5 kg/t of heavy oil. This is because the second reaction zone of the DTFB has the characteristics of high catalyst density and long reaction time, which strengthen the hydride ion transfer reaction. From the reaction mechanism, the hydride ion transfer reaction is the elementary reaction of the hydrogen transfer reaction and the bimolecular cracking reaction. Therefore, strengthening the hydride ion transfer reaction will inevitably increase the hydrogen transfer reaction and the bimolecular cracking reaction rate simultaneously. This strengthens the conversion of small molecular olefins into isoparaffins; on the other hand, it strengthens the bimolecular cracking reaction between small molecular olefins in gasoline and large molecular saturated hydrocarbons in diesel and slurry, which reduces the yield of diesel and slurry. Finally, gasoline yield increases and gasoline olefin content decreases.

Advances in processing high-proportion intermediate-base vacuum resid (VR)

A DTFB allows a wide adaptability of raw materials and can even break through the bottleneck of the selection of catalytic cracking raw materials to realize the long-term safe and stable operation with high-mixing ratio and high-carbon residue raw materials. With the newly developed catalyst rich in intracrystalline mesoporous Y-type molecular sieve, the gasoline yield reaches 44.6% when the catalytic cracking raw material contains 46.6% intermediate-base VR (TABLE 4).

The coke produced by this unit can use the newly-developed 10 MPa-grade steam generation technology to make full use of the characteristics of heavy oil catalytic cracking of a large steam production load and high thermal energy. This can increase the steam pressure of waste heat boilers and external heat extractors to high pressure levels (10 MPa). Therefore, 280 t/hr of high-pressure steam produced by the heavy oil catalytic cracking unit with an annual capacity of 2.8 MMtpy will be sent to the steam turbine unit (25 MW) for power generation through the system pipe network. After the stable operation of the high-pressure steam system, the company’s power center changed. Two 130-t/hr circulating fluidized bed (CFB) coal-fired power boilers were shut down, and the catalytic cracking unit became the company’s new power center.

Takeaway

A DTFB is a multi-flow single-vessel fluidized bed that is organically composed of a transport bed, a fast fluidized bed and a turbulent fluidized bed. It has the characteristics of multi-flow and multi-temperature zones, and has developed into an open reactor engineering platform that is suitable for processing various types of distillate oil or heavy oil with high residue content. On this platform, a series of new technologies, such as a catalytic cracking technology for producing ultra-low olefin gasoline and a catalytic cracking technology for producing more propylene and marine fuel oil components, have been developed, injecting new vitality into catalytic cracking technology development. HP

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