Relationship between olefins and coking propensity of heavy residual oil derived from vacuum residue thermal cracking products

Abstract

Olefins, especially olefinic-bond-containing aromatics (OAH) may have a significant effect on the coking propensity of heavy residual oil (RO) derived from vacuum residue (VR) thermal cracking products in the further deep processing. However, this has been rarely studied in the past. In this work, the structure of RO (boiling point > 500 °C distillate) products and their SARA fractions derived from thermal cracking of VR under different conditions were systematically characterized by 1H nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectroscopy to investigate the occurrence pattern of olefins. The coking propensity of RO in the further processing was evaluated by thermal gravimetric (TG) and coking factor analysis, and the effect of different olefins on the coking propensity of RO in further processing was revealed. The results showed that the thermal cracking led to the occurrence of olefins, especially OAHs in resins (Re) and asphaltenes (Asp). The highest olefinic hydrogen contents of RO and its SARA fractions were detected at the thermal cracking condition of 415 °C for 40 min, which were 2.9 %, 4.0 %, 3.4 %, 2.8 %, and 2.4 %, respectively. Interestingly, RO obtained at the thermal cracking condition of 415 °C for 40 min also had the highest content of TG carbon residue (11.2 wt%) and the largest value of coking factor (5.33 %). Significantly, the TG carbon residue content and coking factor value of RO linearly increased with the total olefinic hydrogen content of RO, suggesting that the occurrence of olefins during thermal cracking significantly contributed to a greater coking propensity of RO in further deep processing. More importantly, an in-depth investigation by adding different olefin model compounds to the original VR thermal reaction system revealed that the olefinic-bond-containing aromatics in Asp may confer the greatest contribution to the increasing coking propensity of RO, followed by the olefinic-bond-containing aromatics in Re.