The literature of the past thirty years shows that the low- temperature oxidation (LTO) of hydrocarbon liquids generally results in a more viscous end product. The In Situ Combustion Research Group at The University of Calgary has found, however, that upgrading can occur during LTO in the presence of caustic additives. Because it was believed that caustic inhibits oxidation reactions as evidenced by a reduction in or absence of coke formation, and allows the oil to upgrade by thermal cracking via a free radical mechanism, a systematic study was undertaken to investigate the effect of caustic on the LTO of heavy oil.
To date, nearly 200 LTO tests have been performed on Athabasca bitumen. These experiments were carried out by varying caustic concentrations, temperatures, oxygen partial pressures, total cell pressures, and run times. All effluent gases were analyzed using gas chromatography, the pH of free water was measured, and hydrocarbon products underwent determination of coke and asphaltene contents, viscosity, and density. CHN and S analyses were carried out on the whole oil, and coke and asphaltene fractions.
Several instances of upgrading were observed. Optimum conditions occurred at lower caustic concentrations, lower temperatures, lower oxygen partial pressures, and longer run times. The amount of oxygen reacted appears to be the most critical parameter affecting the system. The presence of caustic apparently did not inhibit the oxidation reactions from taking place, but, rather, modified the process by impeding the asphaltene fraction from converting to coke.
Introduction
The amount of information available on the LTO of crude oils is relatively limited. Several published studies investigate the chemical and physical changes accompanying LTO of many different grades of oil. Studies on the LTO of Athabasca bitumen described by Babu and Cormack show a decline in the aromatic content, an increase in asphaltenes content, and a stable saturates content. The same authors report a steady increase in coke production with the extent of oxidation. The overall trend is for a conversion from aromatics to resins, from resins to asphaltenes, and from asphaltenes to coke. This transformation to a heavier, more polar oil results in increased viscosities and densities. Millour et al. describe and relate the oxygen uptake to coke deposition, and Severin et al. provide a relationship between viscosity and the percent increase in oxygen concentration in the oxidizing gas. It has been accepted, therefore, that LTO causes undesirable changes in the chemical and physical properties of oil.
It may, however, be beneficial to subject an oil to LTO. Unpublished experimental work performed by co-workers at The University of Calgary showed that a pre-oxidized oil underwent accelerated thermal cracking compared to an unoxidized sample of the same oil when subjected to the same reaction conditions. It was thought that pre-oxidation of the oil incorporated oxygen into the structure, thereby providing labile bonds which more readily generated free-radicals. Introduction of oxygen into the molecular structure of the oil can have two consequences; either a downgraded or an upgraded product.
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