(328g) Coke Formation During Pyrolysis and Oxidation of a Heavy Crude Oil At Atmospheric Pressure

A reaction
mechanism that takes into account the most important chemical pathways that can
produce coke during the rapid heating of heavy crude oil was proposed and contrasted
with experiments using a diffusion-flamelet-based Hencken burner. Coke
formation, a process usually associated with heavy crude oils, is of paramount
importance for the petroleum industry. In some cases coke is an undesired
product as it generates deposits that deactivate catalysts in thermal cracking
units. However, in other processes, e.g. during in situ combustion (ISC), coke
is a desired fuel that can be burned with hot air to increase the temperature
of the reservoir and reduce the viscosity of a heavy crude oil to facilitate
its extraction.

While there are
several studies akin to coke formation in areas such as combustion (soot
formation) and thermal cracking (e.g. ethane cracking), in some areas, such as ISC,
only empirical mechanisms, that do not give a clear explanation of the
chemistry involved, have been proposed. A clear example of this situation is
the mechanism presented by Belgrave¹ in 1993, nowadays the most
accepted in the ISC community, that presents reactions such as
Maltenes→Asphaltenes and Asphaltenes→Coke. Contrary, in other
petroleum fields such as refinery, the mechanisms of pyrolysis and thermal
cracking to describe coke formation and combustion of hydrocarbons involve more
species and more reactions. For instance, the mechanism presented by Ranzi et
al.² in 1979 for the thermal cracking of
hydrocarbons included 85 species and 1351 reactions.

By applying some of the understanding obtained from
other fields related to coke formation, we propose a reaction mechanism that
describes coke formation during the rapid heating of heavy crude oil. During
the pyrolysis of heavy crude oil, polyaromatic molecules grow due to the
addition of radicals present in the crude oil on unsaturated sites, forming
higher-boiling-point aromatics in the presence of oxygen. The
higher-boiling-point aromatics undergo physical condensation forming a
precursor coke deposit. This ?immature? coke deposit grows to form coke as
result of various phenomena: the addition of lower boiling point aromatic
radicals and unsaturated molecules, the continuous physical condensation of
high-boiling-point aromatics and dehydrogenation.

This model of
coke formation was contrasted against experimental results obtained at Sandia's
optical entrained flow reactor that operates at one atmosphere and uses a
diffusion-flamelet-based Hencken burner to provide a high-temperature gas flow.
When a mixture of Ottawa sand and a Colombian heavy crude oil is injected into
the reactor, the high temperature and rich environment, with a low
concentration of oxygen (about 1% molar percent), causes that the crude oil, coating
the Ottawa sand particles, undergoes vaporization, pyrolysis and oxidation.
Samples of the sand/processed-oil mixture, taken at different heights using a
water- and gas-cooled sampling probe, show the advance in the coke-formation
process. SARA analysis and the H/C ratio of the coke give some insight about
the validity of the proposed mechanism.

REFERENCES