Mathematical Modeling of Thermal Conversion of Athabasca Bitumen. Part I: Reaction Pathways and Experimental Method
Authors: Chan K. , Diaz-Goano C. , Dettman H. , De Bruijn T. 
Published: March 2010

Modeling of a thermal cracking process involves establishing a set of kinetic reactions that transform the feed into products. In a typical feed, there exist a large number of real components. Thus, an exact feed composition is not known. The most common method to overcome this difficulty is to define pseudo-components based on physical properties such as boiling point or molecular weight.

In this work, we propose a set of model thermal cracking reactions based on nuclear magnetic resonance (NMR) data. NMR carbon type analysis improves the characterization of the feed and products by providing the contents of different carbon species (carbon bonds). This additional information enables a more descriptive and fundamental set of reactions to be developed for the model. The reactions chosen describe the types of carbon bonds that break and form in the feedstock under visbreaking conditions. The set of reactions is the centre of the pseudo-component model.

Introduction

Visbreaking is a thermal cracking process that has existed for many decades. It is a relatively mild process used primarily to reduce the viscosity of the heavy ends of crude oil feedstocks that results in low conversion of heavy-end material. For example, typically there is &lt; 30 wt% conversion of residue with boiling points (BP) &gt; 524 °C to lower-boiling components. Visbreaking is widely used in Europe and Asia due to the properties of available feedstocks and demand for heavier products such as fuel oil. In Alberta, an increasing proportion of the oil produced is extra heavy (i.e. Athabasca bitumen). Consequently, higher severity processes such as coking and residue hydrocracking are needed to convert 60 wt% or more of the residue to lighter products suitable for transportation fuels. However, there has been some interest in visbreaking as a field upgrading process to reduce the need for diluents to meet pipeline specifications for bitumen and heavy oil transportation.

Despite visbreaking being a relatively simple process, modeling of the thermal cracking reactions that result in product formation poses many challenges. The greatest difficulty is to find a suitable method to describe the feedstock and product molecules. For the lightest components of crude oils (i.e. naphtha, BP &lt;204 °C), there are over a thousand molecular species that can be identified and quantified. However, for fractions with boiling points &gt; 204 °C, virtually every molecule is different. Thus, the challenge is to lump the molecules into a manageable number of groups, yet be able to retain enough chemical sensitivity so as to be able to develop fundamental correlations.

The most common method to characterize a feedstock is to define pseudo-component lumps based on physical properties like molecular weight or boiling point. However, as these properties provide no specific information of the relative chemical reactivity of the lumps, the kinetic parameters determined are empirical and need to be adjusted for each different feedstock.

Recently, nuclear magnetic resonance (NMR) spectroscopy has been used for structural analyses of petroleum fractions. Ali et al.(1) used NMR data to estimate average molecular structures of Kuwaiti vacuum gas oil.