June 2018

Process Optimization

Novel FCC technologies based on reaction chemistry of catalytic cracking

The increased use of opportunity crude oils in refinery processing, the growing demand for light fuel, and the efficient conversion of petroleum resources to refined fuels and basic chemical raw materials have become the basis of green and low-carbon refining technology developments.

Xie C., Wei, X., Gong, J., Long, J., SINOPEC Engineering Inc.

The increased use of opportunity crude oils in refinery processing, the growing demand for light fuel, and the efficient conversion of petroleum resources to refined fuels and basic chemical raw materials have become the basis of green and low-carbon refining technology developments.1,2

Due to its various advantages of low equipment investment, good feed adaptability, high heavy oil conversion and light oil yield, and flexible operation modes, the fluid catalytic cracking (FCC) process continues to play a key role in a modern refinery as the primary conversion process of crude oil into fuels and light olefins. Since the startup of the first commercial FCC unit (FCCU) in 1942, many efforts have been made to enhance the unit’s ability to crack heavier, lower-value feedstocks. In China, approximately 190 FCCUs are operating, representing a total capacity of 210 MMtpy. Catalytic cracking technologies are facing new opportunities and challenges in China due to requirements for high product yields, better fuel quality, more propylene production and low carbon dioxide (CO2) emissions.

Starting from studies on the fundamental theory of catalytic cracking reactions, several proprietary catalytic cracking technologies have been developed to process inferior feedstocks to produce clean fuels and petrochemicals. The following provides a detailed overview of several of these FCC technologies.

Catalytic cracking for petrochemicals production

One such proprietary catalytic cracking technologya (FCC-1) has been developed that converts heavy hydrocarbon feedstocks into light olefins and refined fuels. The first unit was put into operation in 1990.3,4 This technology has been licensed in 18 units in China and overseas markets, with 10 units in operation and eight either being designed or under construction. The world’s largest FCC-1 unit, with a capacity of 92 Mbpd, is located at the Petro Rabigh refinery in Saudi Arabia.

FIG. 1. The reaction path schematic of chain initiation in catalytic cracking of n-hexadecane.

This new catalytic cracking technology was based on research of ethylene and propylene formation mechanisms from the catalytic cracking of heavy oil.5 Protolytic cracking reactions via intermediates of carbonium ions, as well as bimolecular cracking reactions via intermediates of carbenium ions, both contribute to chain initiation. The reaction path schematic of chain initiation in catalytic cracking of n-hexadecane is shown in FIG. 1. The characteristic products of chain reactions vary with chain initiation paths. Dry gas is the characteristic products of protolytic cracking, while C3–C4 olefins are the characteristic products of bimolecular cracking. The tendency of dry gas formation is reduced in the FCC-1 reactions by promoting bimolecular cracking. Propylene selectivity can be improved by suppressing skeletal isomerization, which contributes to iC4 formation. The FCC-1 technology provided propylene yields of up to 28%—increasing by an additional 6% when using the mixture of Daqing vacuum gasoil (VGO) and vacuum residuum as feedstocks.

FCC-1 plus

The precursors of propylene are the olefins in naphtha. The propylene originates from secondary cracking of olefins in naphtha.6,7 Measures commonly taken to enhance propylene generation are to promote secondary cracking of naphtha (such as utilizing two risers or a riser with two reaction zones), recycling naphtha to a reactor, using a zeolite catalyst or additives to selectively crack naphtha, or allowing severe operating conditions. Commercial results show that these measures increased propylene yields, along with higher dry gas yields. The latest studies show that propylene was produced by cracking heavy oil and then by cracking the FCC naphtha.

FIG. 2. Schematic of the novel reactor designed for the FCC-1 plusb process.

The riser in the FCC-1 process houses the catalytic cracking of heavy oil to produce propylene and propylene precursors, with the bed reactor being used for catalytic cracking of naphtha olefins. The riser reactor and the bed reactor require different operating conditions. As a result, it is difficult to optimize the operating conditions in different reactors at the same system, which is the main reason for more dry gas and coke in the FCC-1 process. Consequently, FCC-1 plus technologyb uses a differential control scheme of a riser reactor and fluidized rector (FIG. 2).8 In the FCC-1 plus process, high-temperature and high-activity regenerated catalysts are sent to the bed reactor. The bed temperature is dependent on the regenerated catalyst temperature, but not on the temperature of the main riser. This allows for a decrease in the reaction temperature in the riser, which reduces the production of dry gas.

A test run on the first commercial FCC-1 plus unit showed that dry gas yield decreased from 6.8% to 3.36%, LPG yield decreased from 35% to 25.04%, and the total yields of gasoline and diesel increased from 45.1% to 57.19%.

Maximizing catalytic propylene technology

Commercial results show that enhancing secondary cracking of the gasoline fraction can increase the production of propylene and dry gas yield. With the extension of the reaction time, propylene yield increases first and then decreases. Research on the secondary conversion of propylene in FCC-1 shows that propylene can transform into ethylene, propane, butylene, aromatics and olefins in the gasoline fraction under FCC-1 reaction conditions, especially in a dense-phase fluidized bed reactor via a series of chemical reactions.9 Consequently, secondary conversion reactions in the FCC-1 process consume propylene, which cannot be ignored. The shift of cracking heavy oil from overcracking mode into selectivity cracking mode was proposed, and this became the basis of the novel catalytic propylene technology.c

FIG. 3. Schematic of the combined reactor designed for the catalytic propylenec process.

In the catalytic propylene riser reactor, the conversion of the feedstock and the ratio of monomolecular cracking to bimolecular cracking by adopting proper operating conditions are regulated in the riser, which increases propylene- and olefins-rich gasoline production. In the dense-phase fluidized bed, recycled oil and C4 fractions or light naphtha were injected into the reactor at different positions. First, recycled oil encounters hot regenerated catalysts, then mixes with the C4 or light naphtha before entering a dense-phase fluidized bed reactor. FIG. 3 shows a schematic diagram of the combined reactor designed for this process. The first commercial catalytic propylenec demonstration unit began operations in 2011. In the test run, the propylene yield reached up to 17.05%, the isobutene yield was 5.51% and the dry gas yield was 4.79%. The gasoline’s research octane number (RON) reached 94.6. The cetane index of diesel was 30. The total liquid yield of LPG, gasoline and diesel was 80.23%.10

A catalytic pyrolysis process

Studies showed that ethylene can be generated via both thermal radical cracking and carbocation reaction catalyzed by acid sites of catalyst. In the carbocation reaction system, ethylene contributed to α-cracking of carbonium ions formed by protonation of C-C bond activated by Brønsted acid on the end position. Lewis acid can also contribute to ethylene by activating C-H bonds to form carbenium ions following by β-cracking, and by promoting radical cracking to form ethylene.

Short reaction times were the key parameters to promote ethylene formation. Light olefins and benzene, toluene and xylene (BTX) formations depend on the reaction path of olefins in the gasoline fraction. Olefins in the gasoline fraction crack into ethylene or propylene, or dehydrogenation and cyclization produce BTX in the FCC-1 process; therefore, formation reactions of light olefins and BTX controlled in separate zones were proposed. Based on the FCC-1 technology, the catalytic pyrolysis technologyd involved a new type of reactor. High reaction temperatures and a high catalyst/oil ratio were proposed to maximize ethylene, propylene and BTX selectivity.11 The first commercial catalytic pyrolysis demonstration unit began operations in July 2009. Using Daqing automated topology builder (ATB) as the feedstock, the ethylene and propylene yields, in coproduction mode, were 14.84% and 22.21%, respectively.

Catalytic cracking for clean gasoline production

A novel FCC process for maximizing isoparaffins has been developed. The technologye is based on two reaction zones, and is designed for increasing heavy oil conversion and improving gasoline performance.

In the first reaction zone (similar to a conventional FCC riser reactor), the heavy oil conversion occurs when the oil comes in contact with a catalyst at a high temperature. The second reaction zone is designed for isomerization and hydrogen transfer reactions, to increase isoparaffins content in gasoline. Compared with the conventional FCC process, the isoparaffins FCC technologye has good heavy oil conversion, product distribution and low energy consumption, as well as low sulfur content in gasoline. This technology has been applied to 17 catalytic cracking units in China since its first commercialization in 2002. According to statistical data of existing units, the total liquid yield increased 1.56%, while the sulfur transfer coefficient of gasoline decreased by a significant 27.95%. RON and the motor octane number (MON) of gasoline increased by 0.4 units and 1.2 units, respectively, despite a decline in olefin contents of 14.26%.

Gasoline and propylene production technology

In China, propylene is the most valuable product, followed by gasoline. Based on the study of catalytic cracking reaction mechanisms and acid-catalysis chemistry of heavy oil, a proprietary gasoline/propylene technology has been developed. This technology comprises two reaction zones with different reactor diameters, which can provide moderate operating conditions for monomolecular cracking and bimolecular cracking of heavy oil. The technology utilizes the unique structure and active components of tailor-made catalyst to convert selective hydrocarbons to propylene and isoparaffins-rich gasoline.

In the gasoline/propylene process, produced gasoline can meet Euro 3 standards, and the propylene yield is increased to satisfy petrochemical industry needs, as well. Commercial data showed that the olefins content of gasoline decreased by 14.9%; propylene yield increased by 2.97%; RON and MON of gasoline increased by 1.9 units and 2 units, respectively; dry gas yield decreased by 22.06%; the sulfur content of gasoline decreased by 42.67%; and the total liquid yield increased significantly.

High-octane-number gasoline and light aromatics

With the restructuring of China’s economy, the demand for gasoline grew slightly faster than diesel demand. The oversupply of diesel and the inadequate gasoline production capacity were caused by low consumption growth for diesel. To balance the supply and demand economics of China’s fuel market, producers had to adjust their fuels production.

Based on the chemical composition and structure of light cycle oil (LCO) and the understandings of reaction chemistry of the FCC process, a new technology was developed to produce gasoline with a high octane number and light aromatics.g In this process, the reaction path is designed by integrating selective hydrotreatment with selective catalytic cracking to produce high-octane-number gasoline and light aromatics.

This technology is the combination of hydrotreating and a catalytic cracking unit. First, the LCO fraction is hydrotreated and then sent to the FCCU. The LCO fraction can be converted to high-RON gasoline and/or C6–C8 aromatics by optimizing the operating conditions of the hydrotreating unit and the catalytic cracking unit. The LCO produced in the FCC process is resent to the hydrotreating unit.

This process has two modes of operation. In Mode 1, the full-range LCO is hydrotreated or the LCO is distillated to a light fraction and a heavy fraction. Next, the heavy fraction is hydrotreated. The mixtures of the hydrotreated heavy fraction and light fraction are injected into the FCCU.

FIG. 4. Schematic of the reactor designed for the isoparaffins process.e
FIG. 5. Process flow diagram of a process that produces a highoctane- number gasoline and light aromatics. g

In Mode 2, heavy oil and the hydrotreated LCO, or the hydrotreated light fraction of LCO, are separately injected into the FCCU. The hydrotreated LCO can either be full-range LCO or the heavy fraction of the LCO. Both modes are shown in FIG. 5. Although the operation of the hydrotreating unit would affect the composition of the hydrotreated LCO due to catalyst activity being decayed, commercial trial results for Mode 1 showed that gasoline yield was increased by up to 60%, with a RON of 96.4.

In Mode 2, the LCO yield was reduced by 15%–20%, and the gasoline selectivity rate was increased to 80%. The gasoline yield increased by 13%–16%, with RON increasing by 0.6 units.

Takeaway

The development of refining technologies is driven by the efficient conversion of heavy oil, the quality upgrading of oil products, the market demand for raw chemical materials, and new regulations for the production of low-sulfur fuels. Based on the understandings obtained from FCC reaction chemistry, a new generation of FCC technologies has been developed. These technologies are focused on the production of clean fuels, heavy crude conversion and petrochemicals production. HP

Notes

a. Refers to Sinopec’s Deep Catalytic Cracking (DCC) technology
b. Refers to Sinopec’s DCC-plus technology
c. Refers to Sinopec’s Maximizing Catalytic Propylene (MCP) technology
d. Refers to Sinopec’s Catalytic Pyrolysis Process (CPP) technology
e. Refers to Sinopec’s Maximizing Iso-Paraffins (MIP) technology
f. Refers to Sinopec’s Clean Gasoline and Propylene (CGP) technology
g. Refers to Sinopec’s LTAG process technology

Literature Cited

  1. Dagong, L., “Petroleum industry: Market changes and technical strategy,” ACTA Petrolei Sinica, 2015.
  2. Zhu, H., “Present state and outlook of China’s oil refining industry,” International Petroleum Economics, 2015.
  3. Zaiting, L. and J. Fukang, “Commercial experience of DCC technology,” Petroleum Processing and Petrochemicals, 1991.
  4. Xie, C. and G. Yongcan, “Advances in DCC process and catalyst for propylene production from heavy oils,” China Petroleum Processing and Petrochemical Technology, 2008.
  5. Xie, C., X. Wei and L. Jun, “Molecular reaction chemistry of heavy oil catalytic cracking to propylene,” ACTA Petrolei Sinica, 2015.
  6. Buchanan, J. S., “The chemistry of olefins production by ZSM-5 addition to catalytic cracking units,” Catalysis Today, 2000.
  7. Knight, J. and R. Mehlbert, “Maximize propylene from your FCC unit,” Hydrocarbon Processing, 2011.
  8. Zhang, Z., C. Xie and G. Zhu, “Experimental study of DCC-plus technology,” Petroleum Processing and Petrochemicals, 2010.
  9. Zheng, L., S. Hou and C. Xie, “Propylene transformation during deep catalytic cracking of heavy oil: I. reactivity and reaction pathways,” Acta Petrolei Sinica, 2009.
  10. Xie, C., Y. Gao and R. Yao, “Selective catalytic cracking technology for maximizing catalytic propylene and its commercial application,” Petroleum Processing and Petrochemicals, 2014.
  11. XIE, C. “Studies on catalytic pyrolysis process for ethylene production and its reaction mechanism,” Petroleum Processing and Petrochemicals, 2000.

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