Isoconversional Kinetic Analysis of the Combustion of Heavy Hydrocarbons

Abstract

One method to access unconventional, heavy-oil resources as well as waterflood residual oil is to apply in situ combustion (ISC) to oxidize in place a small fraction of the hydrocarbon, thereby providing heat to reduce oil viscosity and pressure that enhances recovery. Experimental analysis of crude-oil oxidation kinetics provides parameters, such as activation energy, for modeling and optimization of ISC processes. The complex nature of petroleum as a multicomponent mixture and multistep character of oxidation reactions substantially complicates the kinetic analysis of crude-oil oxidation. An isoconversional technique is reported to provide a model-free method for estimating activation energy. The technique naturally deconvolves multistep reactions, providing a useful diagnostic tool that characterizes the combustion qualities of different oils. The experimentally determined combustion kinetics of three crude oil samples, including Hamaca (Venezuela), are explored, adding to the knowledge base of crude-oil combustion. All samples exhibit fairly constant apparent activation energies of about 50 000 J/mol for so-called high-temperature oxidation reactions that occur in the range of 625−700 K. The so-called low-temperature oxidation reactions that occur in the vicinity of 500 K display significant variability and are difficult to group and characterize with a single activation energy. Synthetic examples augment the experimental program and document the diagnostic characteristics of the isoconversional technique applied to crude oil, thereby providing insight into the burning qualities of oil. Phenomena, such as “cool flames”, are identified and marked by negative apparent activiation energy.