(55g) Conceptual Design of the Fluid Catalytic Cracking Process for Maximizing the Yields of Olefins and Aromatics (FMOA)

Fluid Catalytic Cracking (FCC) is one of the core processes in petrochemical refineries. It plays a significant role in converting heavy oil to valuable products such as transport fuels and light olefins ^1,2. For the efficient utilization of oil resources, the high conversion of gas oil is commonly required, while the yields of the dry gas and coke must be restricted at low levels during FCC processes. However, the yields of the dry gas and coke always go up as the conversion of the gas oil increases. Especially when its conversion surpasses 80%, the yields of the dry gas and coke grow rapidly in an exponential manner³. In order to getting rid of high yields of the dry gas and coke, the conversion of gas oil must be maintained below 80% which can be operated under a moderate operating condition. Therefore, some researchers call the process as the moderate catalytic cracking (MCC) process ³. The process is shown as the Case A process in the attached figure. It is obvious that in the MCC process, low yields of the dry gas and coke are obtained at the cost of the relatively low conversion of feedstock. For the purpose of promoting the conversion of the feedstock, the FCC gas oil (FGO), which is made up of the diesel and slurry, can be recycled back to the FCC riser reactor after being hydrogenated. It is believed that with the hydrogenation, the polycyclic aromatic hydrocarbons (PAHs) in the FGO can be converted into the mononuclear aromatics, olefins, or alkanes which can further be cracked readily into light hydrocarbons at moderate operation conditions ⁴. This is the Case B process shown in the attached figure. Furthermore, as we all know, the ethylene and propylene are produced from the cracking reactions of the gas oil, gasoline and butylene during FCC processes. Compared to the gas oil and gasoline, relatively high temperature for the butylene cracking is commonly required, which inevitably gives rise to high yields of the dry gas and coke ⁵. However, if the butylene is oligomerized firstly to be C8 and C12 olefins, the very components of the gasoline, then cracked into the propylene and ethylene in another reactor together with the gasoline olefins, which comes from the gasoline mixtures through solvent extractions, the yields of the dry gas and coke will decrease dramatically as the temperature for the cracking of the gasoline olefins is much lower than that for the butylene. At the meantime, the benzene, toluene and xylene (BTXs) can also be produced with the physical solvent extraction operation. This is the Case C process shown in the attached figure.

Table 1 in the attached figure compares the product yield distributions of the above three FCC processes. Since the operating temperature is a bit lower in the Case A process compared to conventional FCC processes, the conversion of the gas oil is only 78.13%. Meanwhile, the yields of the dry gas and coke are also much lower than that in conventional FCC processes. As for the Case B process in which the hydrogenated FGO is recycled to the FCC riser reactor, while the total yields of the dry gas and coke increase slightly from 8.58% to 9.78%, the yields of gasoline increase dramatically from 33.32% to 47.45%, and the yield of propylene increases from 18.56% to 19.87%. Regarding the Case C process, the yield of BTXs is predicted to be 30%. The estimated yields of the propylene and ethylene are 46% and 4.6%, respectively. Since the gasoline is not a target product for the Case C process, its yield is none, which indicates that FCC units can be designed and constructed to produce only petrochemicals (i.e., propylene, ethene, and BTXs) other than transport fuels. We call the Case C process as the FCC for Maximizing yields of the Olefins and Aromatics (FMOA) process.

The FMOA process integrates technologies of the MCC, FGO hydrogenation, solvent extraction and butylene oligomerization followed by re-cracking. The significance of the FMOA technology is that the flexibility of production plans has changed fundamentally. The FCC process can be designed to produce only basic chemicals (i.e., propylene, ethene, and BTXs).

References

1. Yupeng Du, Hui Zhao, An Ma, Chaohe Yang (2015), “Equivalent reactor network model for the modeling of fluid catalytic cracking riser reactor,” Industrial & Engineering Chemistry Research, 54(35), pp. 8732-8742.
2. Yupeng Du, Lejing Sun, Abdallah Berrouk, et al. (2019), “Novel integrated reactor-regenerator model for the fluidized catalytic cracking unit based on an equivalent reactor network,” Energy & Fuels, 33(8), pp. 7265-7275.
3. Youhao Xu and Shouye Cui (2018), “A novel fluid catalytic cracking process for maximizing iso-paraffins: From fundamentals to commercialization,” Frontiers of Chemical Science and Engineering, 12(1), pp. 9-23.
4. Xin Zhou, Hui Zhao, Xiang Feng, et al. (2019), “Hydrogenation and TMP coupling process: novel process design, techno-economic analysis, environmental assessment and thermo-economic pptimization,” Industrial & Engineering Chemistry Research, 58(24), pp. 10482-10494.
5. Jun Long, Youhao Xu, Jian Zhang, et al. (2011), “Consider new processes for clean gasoline and olefins production advanced technologies promote propylene yield while reducing olefins in gasoline,” Hydrocarbon Processing, 97(9), pp. 85-91.

Acknowledgements

Financial supports from the National Natural Science Foundation of China (21908186), Shandong Provincial Natural Science Foundation (ZR2017LB022), and Shandong Province Higher Educational Science and Technology Program (J17KB075) are acknowledged.