Studies on Coking Reactions of Heavy Oils

Relationships between the Constitution of Heavy Oils and Coking Reactions

Abstract

The pourpose of this paper is to clarify the relation between the constitution of heavy oils and coking reactions. Minas atmospheric residue, its cracked heavy oils and Iranian Heavy vacuum residue were used as samples. The structural parameters for each heavy oils were estimated from the hydrogen distribution determined by ¹H-NMR spectra and elemental analyses,
Kinetic studies were made on coking of these heavy oils in the temperature range from 400 to 500°C.
It was found that the structual parameters showed good correlation with the characterization factor K (Fig. 2).
Assuming that the coking reactions proceed succesively in the manner:
Resin(n_A)→k₂(b)Volatile(n_B)
→k₁(a)Semicoke(n_C)→k₃(c)Coke(n_D)
It was found that the reactions were of the first order with respect to n_A, n_C, and n_D, in which n_A is the yield of the Benzene soluble (BS) fraction, n_C that of the Benzene insoluble/ Quinoline soluble (BI-QS), and n_D that of the Quinoline insoluble (QI) obtained in the course of the reactions.
The corresponding kinetic equations can be written:
dn_A/dt=-(k₁+k₂)n_A (7)
dn_C/dt=k₁n_A-k₃n_C (8)
dn_D/dt=k₃n_C (9)
The rate constant of each step was estimated by using the Gauss-Newton method.
It has been confirmed that the yields of the fractions calculated by the proposed reaction model show good agreement with those obtained experimentally (Fig. 3), and the activation energies and frequency factors obtained from the rate constants exhibit good correlation with the characterization factors (Figs. 5 and 6).
Figs. 5 and 6, suggest that the coking reaction rates can be determined by the number of condensed aromatic rings and by the number of the substituted side chain groups of the heavy oils, and that the characterization factor may be used as one of the indexes of the coking reactions.