December 2022

Refinery-Petrochemical integration

A novel deep catalytic cracking technology for residue-to-chemicals processes

Over the past two decades, annual global demands for refined products and basic petrochemicals have increased by approximately 1.3% and 3.5%, respectively.

Over the past two decades, annual global demands for refined products and basic petrochemicals have increased by approximately 1.3% and 3.5%, respectively. Many refiners around the world are focusing on improving their operations to co-produce petrochemical feedstocks as demand for refined products is forecast to decline. The production of basic petrochemicals accounted for 14 MMbpd of global crude oil demand in 2018 and is forecast to increase to 20 MMbpd by 2050, accounting for approximately 50% of the expected growth in oil demand. In addition, global demand for ethylene and propylene is forecast to increase from 270 MMtpy in 2018 to 385 MMtpy in 2030.

Fluid catalytic cracking (FCC) has become the second major propylene production technology after steam cracking, showing high flexibility in feedstocks and product distribution. FCC units (FCCUs) typically use vacuum distillation products, namely vacuum gasoil (VGO) and vacuum residue as feedstocks. Deep catalytic cracking (DCC) is an FCC process that uses proprietary catalysts to selectively crack various heavy feedstocks to light olefins. The process was developed by the co-authors’ company and has been commercially proven since 1990 through more than 10 installed units. The DCC process is like the traditional FCC process, with the modified reactor consisting of a riser and a fluidized dense bed. The dense bed at the end of the riser results in longer residence times at high catalyst/oil ratios, favoring secondary cracking of primary intermediates, which is believed to increase propylene production at the expense of gasoline yield. It is noteworthy that DCC has poor adaptability to feedstocks. The density of feedstocks is generally not more than 0.92 g/cm ³ , and the hydrogen content is not less than 12.6 wt%. As the quality of the feedstocks tends to be heavier, the application of DCC technology is greatly limited.

Analysis of the DCC reactor

For a long time, research on the DCC reactor ( FIG. 1 ) has been neglected. The proprietary catalyst—consisting primarily of ZSM-5—has a short residence time in the riser, which is insufficient to convert heavy oil to light olefins. Therefore, the combination of a riser and an additional fluidized dense bed is generally considered to be a good choice for DCC focused on enhancing olefin production. Nevertheless, this combination of reactors has several disadvantages, which are discussed below.

The regenerated catalysts are transported to the bottom of the riser for contact with feedstocks. The feedstock then undergoes evaporation and catalytic cracking. Since the riser operates in the dilute phase bed, not all feed molecules can effectively contact the catalysts. Those heavy oil molecules that are not in contact with the regenerated catalysts may undergo unwanted thermal cracking, producing byproducts (such as methane and coke) instead of propylene. When the catalysts enter the fluidized dense bed, some coke has accumulated on the surface or in the pores. Even operating in the dense bed, these partially deactivated catalysts are less capable of further cracking intermediates, such as gasoline to propylene, and may instead lead to more coke formation.

The temperature at the bottom of the riser must be controlled to suppress thermal cracking. The axial temperature drop along the riser can reach more than 50°C (122°F). However, the activation energy to produce propylene rises with the increasing cracking depth of the feedstock. Consequently, the temperature profile within the combined reactor of the DCC unit is inverse to the desired temperature profile for propylene production, as shown in FIG. 2 .

DCC technology is more suitable for processing paraffin-based VGO. With the deterioration of crude oil worldwide, the availability of suitable feedstocks for DCC has gradually reduced. When processing inferior heavy oil, the yields of coke, light catalytic cycle oil, slurry and methane increase significantly. Therefore, new reactors for DCC must be developed to achieve better ethylene, propylene and gasoline yields, even with ultra-inferior feedstocks.

DCC of inferior feedstock in different types of reactors. To develop a new reactor suitable for DCC of inferior feedstocks, the catalytic cracking of an intermediate base hydrotreated residue was evaluated in different types of reactors. The experimental results are shown in TABLE 1 . If the riser’s reactor is used as a benchmark, the product distribution characteristics in the different reactors are the following:

• Riser plus a dense-phase bed: Slightly higher propylene and ethylene yields, but an increasing coke yield of 4.09 wt%
• Dense-phase bed: Relatively higher ethylene and propylene yields, but the highest coke yield of 12.24 wt%
• Fast-fluidized bed: Moderate light olefin and coke yields
• Specially configured fast-fluidized bed: Highest propylene yield of 16.63%, a relatively higher ethylene yield and a moderate coke yield.

The development of residue-to-chemicals technology

The co-authors’ company has developed a novel residue-to-chemicals (RTC) technology ^a to overcome the defect that DCC technology cannot handle heavier and inferior feedstocks. The essential difference between RTC and DCC is that the riser + dense phase reactor in DCC is replaced by a high-efficiency fast-fluidized bed reactor, as shown in FIG. 3 . This RTC technology was successfully commercialized at SINOPEC Anqing Co.’s refinery in January 2020. Compared with DCC, RTC shows good adaptability to inferior feedstock mixed with hydrotreated residue. More importantly, propylene and ethylene can be produced with higher selectivity, while the yields of coke and slurry are reduced.

Technical features of RTC technology

RTC technology exhibits higher selectivity for the conversion of heavy oil to light olefins by applying a fast-fluidized bed reactor. The technology has the following technical characteristics:

Higher catalyst concentration in the RTC reactor. Most FCC reactors use riser reactors by default, ignoring the effect of catalyst concentration on product distribution. However, catalyst concentration plays the most important role in the production of light olefins in DCC. Since the riser reactor operates in the dilute phase bed, some of the feed molecules that are not in contact with the catalyst undergo unwanted thermal cracking to produce ethylene and methane. In contrast, RTC technology uses a fast-fluidized bed with higher catalyst concentrations than conventional risers. The higher catalyst concentration in the reactor means that the feed molecules have a good chance of contacting the catalyst and that more propylene can be produced by β-scission.

Higher temperatures in the RTC reactor. The pre-lift gas transports regenerated high-temperature catalysts to the bottom of the RTC reactor to mix with preheated feedstocks. Although DCC is an endothermic process, the high concentration of catalysts in the reactor can provide sufficient heat capacity for endothermic cracking. Correspondingly, the temperature gradient from the bottom to the top of the RTC reactor axis does not change much. The entire reactor is kept at high temperatures, which is beneficial to strengthen β-scission to produce propylene and suppress hydrogen transfer in DCC.

More efficient contact between catalysts and reactants. Catalytic cracking is a volume expansion process. If the diameter of the reactor remains unchanged, the gas flow velocity gradually increases from the bottom to the top. Severe boundary effects can occur at high central gas velocities. The RTC reactor adopts a special configuration, by which the gas flow velocity along the axial direction of the reactor keeps relatively uniform. Part of the catalyst is refluxed from the boundary to the center of the reactor, enabling more efficient contact between catalysts and reactants.

A longer stable operation period. The rear end of the DCC reactor is a dense-phase bed. The outlet oil gas may diffuse into the whole disengager and eventually form coke. The coke thus formed can easily settle and clog the spent catalyst valve, causing the DCC unit to shut down. In contrast, the outlet of the RTC reactor can be directly connected to an enclosed cyclone to prevent oil gas from diffusing into the disengager to form coke. Therefore, RTC has a longer stable operation period vs. DCC.

Commercial application of RTC technology

In November 2019, SINOPEC Anqing Co. converted its DCC unit to RTC technology, replacing the original combined reactor of a riser + dense-phase bed with a specially configured fast-fluidized bed. During nearly 2.5 yr of smooth operation, test runs (including varying feedstocks and operating modes) were carried out. All tests have demonstrated an increasing light olefin selectivity and a decreasing coke selectivity for the RTC technology. TABLE 2 details a comparison of the performance of RTC and DCC when treating hydrotreated residue and VGO blends. Compared with DCC, RTC technology showed an increased ethylene yield of 0.49%, an increased propylene yield of 2.56% and a decreased coke yield of 0.56%. In addition, the mass ratios of ethylene in dry gas, propylene in LPG and the research octane number (RON) of gasoline were improved.

Economic benefits of RTC technology

At the time of this publication, this RTC technology has been licensed to three refineries with a total processing capacity of 12 MMtpy. Another unit, with a processing capacity of 3 MMtpy, will be commissioned in May 2023, and three additional units will be commissioned in 2024. Based on a comprehensive evaluation of product production, product quality and energy consumption, the incremental profit margin of the RTC technology is $11.94/t of feedstocks vs. DCC technology.

Follow-up development of RTC technology

The RTC technology is characterized by the application of a specially configured fast-fluidized bed to intensify β-scission to convert heavy and inferior feedstocks into light olefins. In the further development of RTC technology, its feedstock will be expanded to light raw materials such as crude oil, naphtha and diesel, among others. According to industry forecasts, ethylene demand will continue to increase over the next decade. Therefore, it is necessary to increase ethylene yield from the existing 5% to more than 10%, while maintaining an appropriate propylene yield. HP

NOTES

^a SINOPEC RIPP’s DCC ^pro residue-to-chemicals technology

REFERENCES

1. Lee, R., “Petrochemicals—The growth area that refiners will need,” Asian Petrochemical Industry Conference, Taipei, May 2019.
2. International Energy Agency (IEA), The Future of Petrochemicals—Towards a More Sustainable Chemical Industry, October 2018, online: https://www.iea.org/reports/the-future-of-petrochemicals
3. Eramo, M., “State of the global chemical industry,” Asian Petrochemical Industry Conference, Taipei, May 2019.
4. Wang, X. Q., C. G. Xie, Z. T. Li and G. Q. Zhu, “Catalytic processes for light olefin production,” Practical Advances in Petroleum Processing, 2006.
5. Gong, J. H., L. Wu, Q. Q. Ma, Z. G. Zhang, X. L. Wei, X. L. Chang and J. S. Zhang, “Commercial application of novel deep catalytic cracking technology for resid to chemicals,” Petroleum Processing and Petrochemicals, 2021.

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