Investigation of kinetics and cracked oil structural changes in thermal cracking of Iranian vacuum residues

Two vacuum residues obtained from Iranian Tehran and Bandar Abbas refineries (T-VR and B-VR) were thermally cracked under various operating conditions (soak time and temperature). The yield distribution of the products and the alteration of the physical properties of the liquid products were evaluated in terms of a change in the operating conditions or the feedstock. It was found that, especially, at higher severities, T-VR tends to give higher yields of liquid product and a lower yield of residual product in comparison with B-VR. The average viscosity, density, refractive index, and molecular weight increased and the H/C ratio decreased upon raising temperature for the cracked oils obtained from both of the feedstocks. The integrals derived in quantitative NMR experiments are suggested to be used as a basis for calculation of paraffinic, olefinic, naphthenic, and aromatic portions of a hydrocarbon sample. The effects of temperature, residence time, and feed type on the composition of the cracked oils are thoroughly investigated on the basis of detailed carbon and proton spectra. A three-lump kinetic model is presented to determine the yields of gas, liquid, and (semi)solid in the upgrading process at hand. The unconvertible portion of the solid is proposed to represent coke or fouling tendency for the residue feedstocks.

► Two vacuum residues from Iranian refineries were pyrolyzed.
► NMR spectra were gathered for the residues and their respective cracked oils.
► Effect of temperature, time, and feed type on cracked oil quality was investigated.
► The assumptions of Arrhenius equation and first-order kinetics are valid.
► Unconvertible portion of the feed was calculated from the kinetic analysis.